Chapter 1: Introduction to Second Language Acquisition

Case Study

Linh is a 19-year-old university student from Vietnam who wants to improve her English for study and future work. She learned English in school for many years, but she still feels nervous when speaking. At home, she studies by herself using videos, mobile apps, and online games in English. She also joins an English club to practice with friends and sometimes speaks with international visitors in her city. Linh notices that she learns differently in each situation. She wonders why natural communication, classroom learning, and self-study affect her progress in different ways. She wants to become a confident multilingual speaker.

Discussion

1. What different ways of learning English can you identify in Linh’s experience?
2. In which situations does Linh seem to learn most effectively, and why?
3. How are classroom learning, self-study, and natural communication different from one another?
4. What challenges may prevent Linh from becoming confident in speaking English?
5. What advice would you give Linh to help her become a more successful multilingual learner?

Introduction

Second language acquisition (SLA) refers to the processes through which learners develop knowledge and use of a language beyond their first language. Research in SLA has shown that success in learning an additional language is shaped by multiple interacting variables rather than by a single cause. Among the most widely discussed influences are age, cognitive factors, social context, and individual differences such as motivation, aptitude, and personality. Contemporary SLA scholarship therefore treats second language development as a multidimensional process involving cognitive, social, and affective dimensions rather than as a purely linguistic one (Ellis, 2008; Dörnyei, 2005).

Age

Age has long been regarded as an important variable in SLA. One influential explanation is the Critical Period Hypothesis, which proposes that there is a biologically favorable window for language learning and that attainment becomes less native-like after that period. In SLA research, the strongest evidence for age effects is often found in pronunciation and ultimate attainment, although the relationship is more complex than a simple early-versus-late distinction. DeKeyser (2000) argues that age effects are robust, especially where implicit acquisition is concerned, while later learners may rely more heavily on explicit learning mechanisms. More recent large-scale evidence also supports the existence of age-related constraints, while showing that the decline in learning ability is gradual and shaped by multiple factors rather than a single cut-off point (DeKeyser, 2000; Hartshorne et al., 2018).

Younger learners are often advantaged in naturalistic settings because extended exposure allows them to build phonological sensitivity and implicit knowledge over time. Research on bilingual development also shows that children can acquire another language successfully through sustained interaction and meaningful exposure, especially when they have rich opportunities to use the language in context. At the same time, age does not determine success on its own. Older learners frequently outperform younger learners in rate of initial learning, particularly in formal instructional settings, because they bring stronger metalinguistic awareness and more developed learning strategies to the task (Paradis et al., 2021; DeKeyser, 2000).

Cognitive development

Cognitive factors also play a central role in SLA. Among these, working memory has received substantial attention because it supports the temporary storage and processing of linguistic information during comprehension and production. Learners with stronger working memory are generally better able to hold lexical and syntactic material in mind while interpreting input, producing output, and noticing language patterns. Wen (2016) argues that working memory is closely related to second language learning and processing, particularly in tasks involving complex comprehension and speech production.

Attention is equally important because learners do not acquire all features of the input automatically. What learners notice in interaction and instruction affects what becomes intake for learning. In this sense, SLA depends not only on the amount of input available but also on the learner’s capacity to attend to relevant linguistic forms and meanings. Ellis (2008) emphasizes that cognitive processing, including attention and memory, mediates how learners convert exposure into developing competence. Thus, cognitive resources influence how efficiently learners process input, test hypotheses, and restructure their interlanguage over time (Ellis, 2008; Wen, 2016).

Social context

SLA is also deeply shaped by the social environment in which learning occurs. Long (1996) argues that interaction is central because it provides learners with access to input, feedback, and opportunities to negotiate meaning. When learners interact with more proficient speakers, they are exposed not only to vocabulary and grammar but also to pragmatic norms and discourse patterns that are difficult to acquire through decontextualized study alone. Social interaction therefore helps connect linguistic knowledge with communicative use (Long, 1996).

Swain (2000) further shows that language development is supported not only by receiving input but also by producing language. Through collaborative dialogue and pushed output, learners may notice gaps in their knowledge, test hypotheses, and reflect metalinguistically on what they are trying to say. In classroom and immersion contexts, authentic communication and meaningful participation can therefore strengthen development by linking comprehension, production, and feedback. For this reason, learners who have frequent opportunities to use the target language in socially meaningful settings often progress more effectively than those whose experience is limited to isolated practice (Swain, 2000; Ellis & Shintani, 2014).

Individual differences

Individual differences remain one of the most important areas in SLA because learners vary substantially even under similar learning conditions. Motivation is especially influential. Dörnyei (2005) argues that motivated learners are more likely to invest effort, persist through difficulty, and seek opportunities to use the target language. Motivation is not static; rather, it is shaped by goals, identity, learning experiences, and the social value attached to the target language. This means that learners’ progress depends not only on cognitive ability but also on how strongly they want to engage with the language over time (Dörnyei, 2005; Dörnyei & Ushioda, 2011).

Language aptitude is another important predictor of achievement. Skehan (1998) describes aptitude as a set of abilities that facilitate language learning, including sensitivity to linguistic patterns, memory for language material, and analytic ability. Learners with stronger aptitude may acquire certain aspects of language more efficiently, especially in formal learning environments. Personality can also shape opportunities for practice. For example, Dewaele and Furnham (2000) found that personality is related to second language speech production, suggesting that traits such as extraversion may support oral participation and fluency in communicative tasks. However, no single learner profile guarantees success, and personality effects often interact with classroom context, task demands, and motivation (Skehan, 1998; Dewaele & Furnham, 2000).

Finally, age-related differences remain relevant within the broader category of individual differences. For example, Flege (1999) shows that age of learning is strongly associated with second language speech, especially accent and phonological accuracy. Yet even here, age interacts with amount of use, quality of input, and experience. Overall, SLA is best understood as the outcome of multiple interacting variables rather than as the direct effect of any one factor. Educators should therefore design instruction that recognizes learner diversity, supports meaningful interaction, and sustains motivation while also accounting for cognitive and developmental differences (Flege, 1999; Ellis, 2008).

Stages of Second Language Acquisition

Second language acquisition is often described in developmental stages to explain how learners gradually move from limited comprehension and production to more fluent and accurate use of the target language. Although these stages are useful for teaching and observation, SLA researchers generally caution that development is not strictly linear and that learners progress at different rates depending on exposure, instruction, age, and learning conditions. In this sense, stage models are best understood as broad descriptions of common patterns in development rather than fixed sequences that all learners follow in exactly the same way (Lightbown & Spada, 2006; Selinker, 1972).

A commonly used description begins with the pre-production or silent period, during which learners focus primarily on listening and comprehension. At this stage, production is very limited, and learners may rely on gestures, short formulaic expressions, or single words rather than full sentences. Krashen and Terrell (1983) associate this phase with the importance of comprehensible input, arguing that learners often need substantial exposure before they can produce language confidently.

The next phase is often described as early production. Here, learners begin to use short utterances, simple sentence patterns, and memorized chunks to communicate basic meanings. Their vocabulary remains limited, and errors are common, but they are increasingly able to respond to simple questions and participate in highly supported interaction. This stage reflects the gradual movement from receptive understanding to initial productive control of the second language (Krashen & Terrell, 1983; Lightbown & Spada, 2006).

As learners gain more control over the language, they enter a stage often called speech emergence. At this point, they can produce longer utterances, express more ideas, and participate in short conversations, although their speech may still contain noticeable grammatical and lexical errors. This stage is closely related to the development of interlanguage, the evolving linguistic system that learners construct as they test hypotheses and gradually approximate target-language norms (Selinker, 1972).

A further stage is commonly labeled intermediate fluency, in which learners become more effective in handling everyday communication and academic or social interaction. Their speech is generally more fluent, their vocabulary is broader, and their control of grammar becomes more stable, even though non-native-like features may remain. At this level, learners are not only producing more language but are also beginning to manage more complex discourse and meaning in context (Lightbown & Spada, 2006).

The final stage is often referred to as advanced fluency. Learners at this level can usually understand and produce complex language with a high degree of effectiveness across a range of contexts. However, SLA research suggests that advanced proficiency should not be equated too quickly with complete native-like competence, since some aspects of pronunciation, grammar, or pragmatic use may continue to develop over a long period or remain variable. What matters more at this stage is the learner’s communicative competence, or the ability to use language appropriately and effectively for different purposes and settings (Canale & Swain, 1980; DeKeyser, 2000).

In brief, describing SLA in stages can help teachers understand learner development and design instruction that matches learners’ current abilities. At the same time, research on interlanguage and communicative competence reminds us that second language development is dynamic, variable, and strongly shaped by the learning environment rather than by a simple step-by-step progression alone (Canale & Swain, 1980; Selinker, 1972).

Multilingual Language Acquisition

Multilingual language acquisition refers to the development of competence in more than one language, whether the languages are acquired from birth or learned sequentially across childhood and later life. In practice, bilingualism is the most frequently discussed form of multilingualism, but the broader concept includes the acquisition and use of three or more languages. Contemporary research emphasizes that multilingual development should not be viewed as a deficient version of monolingual development; rather, it is a normal and widespread outcome of language learning in multilingual families, schools, and societies (Grosjean, 2010; Paradis et al., 2021).

A common distinction in this literature is between simultaneous bilingualism and sequential bilingualism. Simultaneous bilingualism refers to learning two languages from birth or very early childhood, whereas sequential bilingualism occurs when a second language is introduced after the first language has already been established. This distinction is useful because the timing and amount of exposure influence how languages develop, although both pathways can lead to high levels of bilingual competence. Research on dual language development shows that bilingual children generally follow the same broad developmental path as monolingual children, but their progress in each language depends heavily on the quantity and quality of input they receive in that language (Hoff et al., 2012; Paradis et al., 2021).

Research has also shown that bilingual development is not simply the sum of two separate monolingual systems. Bilingual learners draw on both languages in dynamic ways, and their language use may include cross-linguistic influence and code-switching. These patterns are normal features of bilingual competence rather than signs of confusion. Grosjean (2010) argues that bilinguals should be understood as whole language users whose linguistic repertoires are shaped by context, need, and experience, not by monolingual norms.

Another major theme in the literature concerns the potential benefits associated with bilingualism and multilingualism. Bialystok’s work has linked bilingual experience with aspects of executive functioning and metalinguistic awareness, although the size and consistency of these advantages may vary across studies and populations. What is more firmly supported is that bilingual learners often develop heightened sensitivity to language as a system and increased flexibility in managing more than one linguistic code (Bialystok, 2015).

At the same time, multilingual development is shaped by important contextual factors, including age of acquisition, continuity of exposure, family language practices, schooling, and opportunities to use each language meaningfully. Bilingual children are not expected to show identical vocabulary size or grammatical growth in each language at every moment; rather, their development reflects how experience is distributed across their languages. For this reason, bilingual performance should be interpreted in relation to bilingual exposure and use, rather than measured solely against monolingual benchmarks (Hoff et al., 2012; Paradis et al., 2021).

Overall, multilingual language acquisition is best understood as a dynamic developmental process shaped by input, use, and social context. Although learners may differ in how their languages develop, multilingualism is a legitimate and valuable form of language competence that can support linguistic flexibility, broader communicative repertoires, and participation across multiple cultural settings (Grosjean, 2010; Paradis et al., 2021).

The Role of Age

Age is one of the most widely discussed variables in second language acquisition (SLA). A major theoretical foundation for this discussion is the Critical Period Hypothesis, which proposes that language learning is biologically constrained and is generally more successful when it begins earlier in life. Lenneberg’s work is often cited as the starting point for this position, and later SLA research has examined whether age-related constraints apply not only to first language development but also to second language learning. Studies on age effects suggest that earlier exposure is often associated with higher levels of ultimate attainment, especially in pronunciation and morphosyntax, although this relationship is not absolute (Lenneberg, 1967; DeKeyser, 2000; Johnson & Newport, 1989).

Research on second language learning has shown that younger learners often have an advantage in long-term attainment, particularly in naturalistic settings with sustained exposure. Johnson and Newport’s well-known study found a strong relationship between age of arrival and performance in English among immigrants who began learning the language at different ages, supporting the view that maturational factors matter in SLA (Johnson & Newport, 1989). At the same time, later learners are not excluded from success. Older learners may progress more rapidly in the early stages of instruction because they can draw on more developed cognitive, metalinguistic, and problem-solving abilities (DeKeyser, 2000). For this reason, age should be understood as an important but non-deterministic variable that interacts with input, context, and learning conditions (DeKeyser, 2000).

Bilingualism

Bilingualism refers to the regular use of two languages, and it is one of the most common forms of multilingualism. Contemporary research emphasizes that bilinguals should not be evaluated against monolingual norms alone, because bilingual language knowledge is distributed across languages and shaped by patterns of use, context, and experience. Grosjean argues that bilinguals are whole language users whose linguistic repertoires reflect the demands of their social worlds rather than two separate monolingual systems housed in one person (Grosjean, 2010). This perspective is important because it shifts attention from deficit views of bilingualism to a more realistic understanding of bilingual competence (Grosjean, 2010).

Bilingualism can develop simultaneously, when two languages are acquired from birth or very early childhood, or sequentially, when a second language is added after the first has already been established. In both cases, development depends heavily on the quantity and quality of input, opportunities for meaningful use, and continuity of exposure across home, school, and community settings. Research on dual language development shows that bilingual children generally follow normal developmental pathways, but their growth in each language reflects how experience is distributed across those languages rather than mirroring monolingual development in each language separately (Hoff et al., 2012; Paradis et al., 2021).

Bilingualism has also been linked to metalinguistic awareness and aspects of executive control, although the strength and consistency of these advantages vary across studies. Bialystok’s work has been especially influential in showing that bilingual experience can shape attention, cognitive flexibility, and language awareness (Bialystok, 2015). Even so, the most defensible conclusion is not that bilingualism automatically produces cognitive superiority, but that managing more than one language may influence how individuals attend to language and control competing information (Bialystok, 2015).

Cognitive Development

Cognitive development is closely related to SLA because language learning depends on the learner’s ability to process, store, and retrieve linguistic information. Among the cognitive factors most frequently discussed in SLA research are working memory, attention, and the capacity to manage complex input during comprehension and production. These abilities help learners retain new vocabulary, process grammatical relationships, and integrate meaning across stretches of discourse (Wen, 2012; Wen et al., 2017).

Working memory has received particular attention because it supports the temporary storage and manipulation of information during language use. Research synthesized by Wen shows that working memory is closely associated with second language learning and processing, especially in tasks that require learners to hold linguistic material in mind while interpreting input or planning output (Wen, 2012; Wen et al., 2017). From this perspective, cognitive development matters not because it guarantees success, but because it affects how efficiently learners can notice patterns, rehearse information, and build more stable linguistic representations over time (Wen, 2012).

In short, cognitive development should be viewed as a facilitating condition in SLA rather than a fixed predictor of achievement. Learners differ in the cognitive resources they bring to language learning, but these resources interact with instruction, practice, and exposure. As a result, successful SLA depends not only on cognitive capacity itself but also on whether the learning environment supports attention, repetition, meaningful use, and gradual restructuring of linguistic knowledge (Wen, 2012; Wen et al., 2017).

Different Types of Second Language Acquisition

Second language acquisition (SLA) can occur in different learning contexts, and these contexts shape both the process and the outcomes of language development. Rather than treating SLA as a single uniform phenomenon, research often distinguishes between naturalistic acquisition, classroom acquisition, and heritage language acquisition. These categories are useful because learners differ in the kind of input they receive, the amount of explicit instruction available, and the purposes for which they use the target language (Ellis, 1989; Lightbown & Spada, 2013; Valdés, 2005).

Naturalistic Acquisition

Naturalistic acquisition refers to second language development that takes place primarily through meaningful exposure and everyday communication rather than through formal instruction. Learners in naturalistic settings acquire language by participating in authentic interaction, interpreting messages, and gradually building communicative ability through use. This type of acquisition is especially associated with immersion in the target-language environment, where learners encounter the language in social, professional, or community life on a regular basis (Ellis, 1989).

Research comparing classroom and naturalistic learning suggests that both contexts may follow broadly similar developmental routes for some grammatical features, although the rate and conditions of learning may differ. Naturalistic acquisition is often especially important for the development of fluency, pragmatic competence, and socially appropriate language use because learners must use the language for real purposes in real contexts (Ellis, 1989).

Classroom Acquisition

Classroom acquisition refers to SLA that occurs in formal instructional settings such as schools, universities, or language institutes. In this context, learning is typically more structured, sequenced, and guided by explicit pedagogical goals. Classroom learners often receive direct support in grammar, vocabulary, literacy, and task performance, and they benefit from systematic feedback and planned progression through levels of difficulty (Lightbown & Spada, 2013).

A major strength of classroom acquisition is that it can make language features more noticeable and provide learners with explicit explanations that may not be available in naturalistic settings. At the same time, classroom learning may offer fewer opportunities for sustained authentic interaction unless instruction is carefully designed to promote communication and contextualized language use. For this reason, many SLA scholars argue that effective classroom learning should combine structured instruction with opportunities for meaningful communication (Ellis, 1989; Lightbown & Spada, 2013; Kramsch, 1993).

Heritage Language Acquisition

Heritage language acquisition refers to the development or maintenance of a language that is spoken in the learner’s home or cultural community and is different from the dominant societal language. Heritage learners often have some degree of prior exposure to the language through family interaction, but their proficiency may be uneven across speaking, listening, reading, and writing. As a result, heritage language acquisition differs from both foreign language learning and monolingual first language development (Valdés, 2005; Polinsky, 2020).

Research on heritage learners shows that they often bring valuable pre-existing linguistic knowledge to the classroom, but they may also have distinctive instructional needs. Their development is shaped by home use, community support, literacy experience, and opportunities to maintain the language over time. Kondo-Brown (2005), for example, shows that heritage learners are not a uniform group and may differ substantially in proficiency and use patterns. This means that heritage language instruction is most effective when it builds on learners’ existing competencies while also addressing gaps in formal registers, literacy, and academic language use (Kondo-Brown, 2005; Valdés, 2005).

These forms of SLA should not be viewed as completely separate or fixed categories, since many learners move across contexts over time. A learner may begin in a heritage language environment, continue through classroom instruction, and later develop greater proficiency through naturalistic immersion. Even so, distinguishing among naturalistic, classroom, and heritage language acquisition helps clarify how different learning environments influence input, interaction, and language outcomes. This distinction is useful for both researchers and educators because it supports more appropriate expectations and more responsive teaching practices (Ellis, 1989; Lightbown & Spada, 2013; Valdés, 2005).

Autonomous Language Acquisition

Autonomous language acquisition refers to language learning in which learners take substantial responsibility for planning, monitoring, and evaluating their own progress. In this view, autonomy does not mean learning in complete isolation; rather, it refers to the learner’s capacity to make informed decisions about goals, materials, strategies, and self-assessment within or beyond formal instruction (Holec, 1981; Benson, 2011).

In SLA research, learner autonomy is generally treated as an important condition that can strengthen long-term language development, especially when learners are able to regulate their study, reflect on progress, and sustain motivation over time. Benson (2011) argues that autonomy is closely linked to control over learning management, learning content, and cognitive processes, while Holec (1981) emphasizes the learner’s ability to assume responsibility for learning decisions. From this perspective, autonomous language acquisition is not a separate mechanism of acquisition so much as a mode of learning characterized by self-direction and active learner agency (Holec, 1981; Benson, 2011).

Autonomous learning can occur in self-access environments, out-of-class study, online learning, and independent practice, but its effectiveness depends on the learner’s ability to select appropriate strategies and engage consistently with meaningful input and use. For that reason, autonomy is often associated with successful language learning, but it should not be assumed to benefit all learners equally under all conditions. It is most productive when learners have sufficient support, resources, and metacognitive awareness to manage their own learning effectively (Benson, 2011).

Incidental Acquisition

Incidental acquisition refers to language learning that occurs as a by-product of engaging with language for purposes other than deliberate study. In SLA, this concept is often discussed in relation to vocabulary growth, input processing, and the role of attention. Learners may acquire lexical items, patterns, or pragmatic features while reading, listening, or interacting, even when their main goal is communication or comprehension rather than explicit learning (Hulstijn, 2001; Schmidt, 1990).

Research on incidental learning suggests that exposure alone can contribute to second language development, especially when learners encounter language repeatedly in meaningful contexts. However, SLA research also shows that incidental learning is constrained by attention and noticing. Schmidt’s noticing hypothesis argues that features in the input are unlikely to become intake unless learners notice them, while Hulstijn (2001) shows that incidental learning may be beneficial but does not eliminate the role of intentional study, rehearsal, or deeper processing. As a result, incidental acquisition is best understood as a valuable contributor to language growth rather than a complete substitute for focused learning (Hulstijn, 2001; Schmidt, 1990).

For this reason, incidental acquisition is often most effective when combined with other forms of learning. It can support vocabulary expansion, comprehension development, and familiarity with usage patterns, but by itself it may not be sufficient for learners who need precise control of grammar, academic language, or advanced productive skills. In pedagogical terms, incidental acquisition is therefore important not because it replaces instruction, but because it complements more intentional forms of language learning (Hulstijn, 2001; Schmidt, 1990).

Problem-Solving Task

Linh has studied English for many years, but she still lacks confidence in speaking. She learns English in class, studies by herself with apps and videos, and sometimes talks with foreigners. However, she feels that her progress is slow, especially in speaking and using English naturally.

Task:
Work in groups and suggest a practical plan to help Linh improve her English speaking skills over the next three months.

Your solution should include:

• which type of language learning she should focus on more
• what activities she should do each week
• how she can improve both confidence and fluency
• how her teacher, friends, or family can support her

After discussing, present your group’s solution to the class.

Guiding questions for reviewing the lesson

1. What is second language acquisition?
2. Name three factors that influence second language acquisition.
3. How does age affect language learning?
4. What is the main idea of the Critical Period Hypothesis?
5. Why are older learners still able to succeed in SLA?
6. What is the difference between simultaneous bilingualism and sequential bilingualism?
7. Why is it not accurate to compare bilingual learners only with monolingual learners?
8. How does cognitive development help language learning?
9. What is the role of working memory in SLA?
10. What is the difference between naturalistic acquisition and classroom acquisition?
11. What is heritage language acquisition?
12. How can learner autonomy and incidental learning support language development?

References

Benson, P. (2011). Teaching and researching autonomy in language learning (2nd ed.). Routledge. https://doi.org/10.4324/9781315833767

Bialystok, E. (2015). Bilingualism and the development of executive function: The role of attention. Child Development Perspectives, 9(2), 117–121. https://doi.org/10.1111/cdep.12116

Canale, M., & Swain, M. (1980). Theoretical bases of communicative approaches to second language teaching and testing. Applied Linguistics, 1(1), 1–47. https://doi.org/10.1093/applin/I.1.1

DeKeyser, R. M. (2000). The robustness of critical period effects in second language acquisition. Studies in Second Language Acquisition, 22(4), 499–533. https://doi.org/10.1017/S0272263100004022

Dewaele, J.-M., & Furnham, A. (2000). Personality and speech production: A pilot study of second language learners. Personality and Individual Differences, 28(2), 355–365. https://doi.org/10.1016/S0191-8869(99)00106-3

Dörnyei, Z. (2005). The psychology of the language learner: Individual differences in second language acquisition. Lawrence Erlbaum Associates. https://books.google.com/books/about/The_Psychology_of_the_Language_Learner.html?id=Z5yK9rZklOoC

Dörnyei, Z., & Ushioda, E. (2011). Teaching and researching motivation (2nd ed.). Pearson. https://books.google.com/books/about/Teaching_and_Researching_Motivation.html?id=zNl-RAAACAAJ

Ellis, R. (2008). The study of second language acquisition (2nd ed.). Oxford University Press. https://www.scirp.org/reference/referencespapers?referenceid=2277819

Ellis, R., & Shintani, N. (2014). Exploring language pedagogy through second language acquisition research. Routledge. https://www.taylorfrancis.com/books/mono/10.4324/9780203796580/exploring-language-pedagogy-second-language-acquisition-research-rod-ellis-natsuko-shintani

Ellis, R. (1989). Are classroom and naturalistic acquisition the same? A study of the classroom acquisition of German word order rules. Studies in Second Language Acquisition, 11(3), 305–328.

Flege, J. E. (1999). Age of learning and second language speech. In D. Birdsong (Ed.), Second language acquisition and the critical period hypothesis (pp. 101–131). Lawrence Erlbaum Associates. https://www.scirp.org/reference/referencespapers?referenceid=3341946

Grosjean, F. (2010). Bilingual: Life and reality. Harvard University Press.

Hoff, E., Core, C., Place, S., Rumiche, R., Señor, M., & Parra, M. (2012). Dual language exposure and early bilingual development. Journal of Child Language, 39(1), 1–27. https://doi.org/10.1017/S0305000910000759

Hartshorne, J. K., Tenenbaum, J. B., & Pinker, S. (2018). A critical period for second language acquisition: Evidence from 2/3 million English speakers. Cognition, 177, 263–277. https://doi.org/10.1016/j.cognition.2018.04.007

Holec, H. (1981). Autonomy and foreign language learning. Pergamon.

Hulstijn, J. H. (2001). Intentional and incidental second-language vocabulary learning: A reappraisal of elaboration, rehearsal and automaticity. In P. Robinson (Ed.), Cognition and second language instruction (pp. 258–286). Cambridge University Press.

Johnson, J. S., & Newport, E. L. (1989). Critical period effects in second language learning: The influence of maturational state on the acquisition of English as a second language. Cognitive Psychology, 21(1), 60–99. https://doi.org/10.1016/0010-0285(89)90003-0

Krashen, S. D., & Terrell, T. D. (1983). The natural approach: Language acquisition in the classroom. Pergamon Press.

Kramsch, C. (1993). Context and culture in language teaching. Oxford University Press.

Kondo-Brown, K. (2005). Differences in language skills: Heritage language learner subgroups and foreign language learners. The Modern Language Journal, 89(4), 563–581. https://doi.org/10.1111/j.1540-4781.2005.00330.x

Lenneberg, E. H. (1967). Biological foundations of language. Wiley.

Lightbown, P. M., & Spada, N. (2006). How languages are learned (3rd ed.). Oxford University Press

Lightbown, P. M., & Spada, N. (2013). How languages are learned (4th ed.). Oxford University Press.

Long, M. H. (1996). The role of the linguistic environment in second language acquisition. In W. C. Ritchie & T. K. Bhatia (Eds.), Handbook of second language acquisition (pp. 413–468). Academic Press. https://www.scirp.org/reference/referencespapers?referenceid=476624

Polinsky, M. (2020). Understanding heritage languages. Bilingualism: Language and Cognition, 23(1), 4–20.

Paradis, J., Genesee, F., & Crago, M. B. (2021). Dual language development and disorders: A handbook on bilingualism and second language learning (3rd ed.). Brookes Publishing. https://brookespublishing.com/wp-content/uploads/2021/01/Paradis-3e_First-page-proofs-excerpt.pdf

Selinker, L. (1972). Interlanguage. IRAL: International Review of Applied Linguistics in Language Teaching, 10(1–4), 209–231. https://doi.org/10.1515/iral.1972.10.1-4.209

Skehan, P. (1998). A cognitive approach to language learning. Oxford University Press. https://scholar.google.com/citations?hl=en&user=PkbCnrQAAAAJ

Schmidt, R. W. (1990). The role of consciousness in second language learning. Applied Linguistics, 11(2), 129–158. https://doi.org/10.1093/applin/11.2.129

Swain, M. (2000). The output hypothesis and beyond: Mediating acquisition through collaborative dialogue. In J. P. Lantolf (Ed.), Sociocultural theory and second language learning (pp. 97–114). Oxford University Press. https://www.scirp.org/reference/referencespapers?referenceid=3997241

Valdés, G. (2005). Bilingualism, heritage language learners, and SLA research: Opportunities lost or seized? The Modern Language Journal, 89(3), 410–426.

Wen, Z. E. (2016). Working memory and second language learning: Towards an integrated approach. Multilingual Matters. https://scholar.google.com/citations?hl=en&user=NLHcOBMAAAAJ

Wen, Z. (2012). Working memory and second language learning. International Journal of Applied Linguistics, 22(1), 1–22. https://doi.org/10.1111/j.1473-4192.2011.00299.x

Wen, Z., Li, S., Schwieter, J. W., & Pino, M. A. (2017). Working memory in L2 learning and processing. Studies in Second Language Learning and Teaching, 7(3), 377–396. https://doi.org/10.14746/ssllt.2017.7.3.1

Case Study Linh is a 19-year-old university student from Vietnam who wants to improve her English for study and future work. She learned English in school for many years, but she still feels nervous when speaking. At home, she studies by herself using videos, mobile apps, and online games in English. She also joins an…