Input exposure is essential for nonprimary language learning, but the importance of explicit instruction and corrective feedback continues to be debated. If instruction is required, how might it be optimized in terms of its nature and timing? In this study, 65 Spanish-English bilinguals were introduced to Latin through an interactive computer program designed to promote initial learning of thematic role assignment by drawing learners' attention to efficient input processing strategies when new, informative morphosyntactic cues to thematic role assignment were set up in competition with more familiar but potentially misleading cues. Participants in 4 experimental groups completed input-based, task-essential practice with interpreting agent/patient roles in Latin sentences and received feedback throughout the practice session. Two of the groups additionally received prepractice explanation of how thematic roles are assigned in Latin via morphosyntactic cues. Groups also varied systematically in whether they received less or more explicit feedback during practice. Results suggest that practice and less explicit feedback were sufficient to trigger improvement in the ability to interpret Latin case morphology; however, more explicit, metalinguistic feedback was necessary to promote improvement in production. Prepractice explanation without metalinguistic feedback during practice did not significantly influence development of either interpretive or productive abilities. (Contains 3 figures, 4 tables, and 5 notes.)