International students make up 5.5% of the student body in the US, but little is known about their language development and use. The goals of this study were twofold. First, I aimed to systematically track the amount and types of language use reported by international students immersed in the target language environment. By doing so, I hoped to provide answers to questions such as what types of language skills receive the most (least) attention during daily life as an international student? How much individual variation exists in English use among international students in the US context? Second, I examined the longitudinal associations between two types of second language (L2) knowledge (i.e., explicit knowledge and implicit knowledge) and their association to activities types that invite different types of processing (i.e., language-focused and meaning-focused). The exploration of the knowledge-knowledge associations and the processing-knowledge associations will inform our understanding of the interface question, which concerns how awareness of linguistic form may impact L2 learning (e.g., DeKeyser, 2007; N. Ellis, 2002, 2003, 2005; Hulstijn, 2002; Krashen, 1985; Paradis, 2009). One hundred and twenty-two L2 English learners completed five linguistic tests that measured their explicit and implicit knowledge of L2 English at two-timepoints (T1: January-February 2019, T2: April-May 2019). The untimed written grammaticality judgment test (GJT) and metalinguistic knowledge test served as measures of explicit L2 English knowledge. The timed written GJT, oral production, and elicited imitation were administered as implicit L2 English knowledge measures. To track language engagement, participants completed self-reported language exposure logs on five days over the course of one semester. Using a combination of confirmatory factor analysis and path analysis, I observed that explicit and implicit knowledge are in a reciprocal relationship that affected each other bi-directionally; that is, explicit knowledge at Time 1 was causally related to implicit knowledge at Time 2; and reversely, implicit knowledge at Time 1 played a facilitative role in the development of explicit knowledge at Time 2. Neither of the activity types predicted knowledge development. In addition, data on authentic language usage showed that international students are more engaged (quantitatively, in terms of hours per day spent) with L2 English than other languages. To be precise, they spend 2.2x more time using English than other languages. I also observed qualitative differences in English engagement: While students spent a comparable amount of time speaking, listening, and reading in English, they spent significantly less time writing in English. Lastly, at the individual-level, students showed wide-ranging variability in the amount and types of language engagement they reported.The findings of this dissertation suggest that first, language acquisition is a developmental process composed of a dynamic interaction between explicit and implicit knowledge and their synergetic relationship; and second, similar affordances to engage in the L2 do not produce comparable amounts of actual L2 engagement for different individuals. These observations may reinforce that the explicit-implicit interface question, and language acquisition more generally, maybe better understood when studied over time in a naturalistic context, as language acquisition in its essence is shaped by one's experience with the language in interaction with the contextual affordances in the environment.
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