The article summarizes results from two experimental studies (N = 23, N = 21) investigating the extent to which working memory capacity (WMC) intervenes in "ab initio" language development under two pedagogical conditions [± grammar lesson + input-based practice + explicit feedback]. The linguistic target is the use of morphosyntax to assign semantic functions in Latin. Results suggest that with the more traditional pedagogical approach [+ grammar lesson], WMC does not predict learner outcomes. In contrast, in an approach that immediately immerses learners in meaning-focused practice with the same explicit feedback, WMC predicts gains on interpreting aural and written input. Thus, it appears that pre-practice grammar explanation "levels the field" for learners of varying WMC, while WMC may play more of a role when metalinguistic information is limited to reactive feedback. These results extend previous research (Robinson 2002, 2005b; Erlam 2005; Goo 2012; Li 2013; Sagarra and Abbuhl 2013a, 2013b; Tagarelli "et al." 2014), and further call attention to the importance of interactions between pedagogical tools and individual differences in explaining language development.