This study analyzed how six Heritage language learners at the university level gained conscious awareness and control of the concept of modality as revealed in student verbalizations (Vygotsky, 1998) throughout five different written communicative events. This work took place in the only course designed for Heritage language learners at a large public suburban university in the Northeast part of the United States. Grammatical simplification in bilingual speakers is due to incomplete acquisition of Spanish, attrition or loss of an underused linguistic system (Lynch, 1999; Martinez Mira, 2009a, 2009b; Mikulski, 2010b; Montrul, 2007; Ocampo, 1990; Silva-Corvalan, 1990, 1994a, 1994b, 2003; Studerus, 1995). The result of the process of simplification is reduction or loss of forms and/or meanings. In this work, I investigated in which ways Gal'perin's (1989) systemic-theoretical organized instruction promoted awareness, control and internalization of the concept of modality in three sets of data: definition, discourse and verbalization (Negueruela, 2003). In addition, I examined how the concept of modality emerged and proceeded. By focusing students' attention in Negueruela's (2003) Concept of Mood in Spanish orienting chart in a top down fashion, students were able to strengthen their theoretical understanding in practical activity while still accessing empirical knowledge, and eventually generalizing its use in new contexts across nominal, adjectival and adverbial clauses. At the definition level, Gal'perin's Systemic-theoretical instruction promoted emergence and progress of their conceptual understanding from perceptual to semantic. At the discourse level, students' theoretically based semantic understanding had a positive impact as revealed in student's discourse progress throughout tasks. At the verbalization level, semantic, abstract and systematic verbalizations showed students' emergence of awareness of the interrelated categories of modality. The conceptual category of anticipation was appropriately verbalized and contextualized 68% of the time. The absence of quality verbalizations referring to a specific conceptual category in some students lead me to conclude that students did not fully understand the meaning of some conceptual categories. On the contrary, their presence in any of the tasks showed emergence of conceptual meaning(s) in appropriate contexts, further appropriate recontextualization may provide full awareness and control. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]