This study explores the responsiveness of an elementary teacher's support for emergent bilingual students by examining the connections between the teacher's assessment expertise, formative assessment practices, and emergent bilinguals' unique language needs in math classes. The study, which investigated the indicators that characterize a linguistically responsive and equitable formative assessment, included and prioritized the perspectives of emergent bilingual students. Primary data sources included classroom observations, teacher interviews, and student interviews. This study elucidates pedagogical practices that are beneficial for emergent bilinguals. It found that both emergent bilinguals and teachers generally agree on the best practices, including their understanding of the language support. However, at times, there is a discrepancy between what the teacher said and the support that emergent bilingual students need. This study contributes to a comprehensive understanding of what and how teachers should know and act for performing responsive formative assessments for emergent bilinguals. It has implications for further research and for teachers who work with emergent bilingual students in core content areas.