|Abstract:||The role of attention and awareness is central to second language acquisition development, as proposed by several theoretical accounts (Schmidt, 1993, 2001; Tomlin & Villa, 1994; VanPatten, 1993, 1996) and shown by experimental research (e.g., Godfroid & Schmidkte, 2013; Rosa & O’Neill, 1999; Rosa & Leow, 2004a, 2004b;VanPatten & Cadierno, 1993). The current dissertation aims to contribute to this line of research by examining the effects of language acquisition on L2 learners’ attention allocation, input processing, and learning outcomes. First, this dissertation tests the Lexical Preference Principle proposed in VanPatten’s Input Processing Model (1993, 1996), which states that L2 learners are more likely to rely on lexical rather than inflectional cues. Second, the current study aims to examine if language instruction that takes into account the psycholinguistic processes that govern L2 learners input processing, can help L2 learners overcome this lexical processing bias and pay more attention to morphological cues as they read the input in real time. Third, this study investigates the effects of language instruction on Spanish L2 learners’ interpretation, production, and input processing of the Spanish subjunctive in adverbial clauses. The results from the present study confirmed that Spanish L2 learners did rely more on lexical than inflectional cues at the pretest stage. This was measured by accuracy scores on two interpretation tests and provides support for the Lexical Preference Principle. In addition, the current study found that language instruction can help L2 learners pay more attention to inflectional cues, as measured via eyetracking (Total Dwell Time reading measures); however, this only occurred in one of the two interpretation tests employed. Furthermore, the present study also found that language instruction led to L2 learners’ interpretation and production learning gains, as measured by their responses to two interpretation and one production test before and after instruction. These findings were consistent with previous research that examined the effects of language instruction with this particular target form (Bowles & Henshaw, 2015; Lee & McNulty, 2013; McNulty-Díaz, 2017). Finally, the results from this study found partial support for the claim that language instruction that takes into account the psycholinguistic processes that govern L2 learners input processing, can help L2 learners become more sensitive to grammaticality manipulations of the target form as L2 learners read for comprehension. This study used a sentence reading comprehension test with eyetracking to examine L2 learners’ changes in processing before and after instruction. This dissertation’s results only provide partial support for this claim, as a grammaticality effect in the direction expected (higher reading time for ungrammatical than grammatical items) in the critical region(s) was only found with one of the four eyetracking measures (i.e., with Total Dwell Time but not with First Fixation Duration, Regressions in or Regressions out). In addition to these three main objectives, the current dissertation contributes to previous research on Spanish mood that examined L2 learners’ knowledge of this construct with only an interpretation, production, or a sentence reading comprehension (with eyetracking) test. I investigate L2 learners’ understanding of the Spanish subjunctive in adverbial clauses with a thorough methodology that tests L2 learners’ interpretation, production, and input processing.