Two-way immersion (TWI) programs in the US are not reaching their minority language learning potential. While English home language (EHL) students develop functional proficiency, their minority language (e.g., Spanish) remains grammatically inaccurate. Spanish home language (SHL) students may also develop non-native-like aspects. Scholars suggest that underdeveloped language proficiency is partly due to teachers' tendency to neglect language during content instruction, and form-focused instruction (FFI) has been suggested as a possible remedy. The purpose of this descriptive, observational case study was to investigate how EHL and SHL students responded to FFI focused on past tense-aspects: preterit and imperfect. Data were collected in a 5th grade TWI classroom in a large, urban district in the Midwest. They included 16 audio-recorded classroom observations (5 pre-FFI, 7 during-FFI, and 4 post-FFI), field notes, and teacher interviews. For this study, pre-FFI and post-FFI transcripts were examined for 10 focal students. Data were analyzed both quantitatively (with an obligatory occasion analysis) and qualitatively. Qualitative analysis provides further interpretation of quantitative results.