ASSISTments is a free online learning tool for improving students' mathematics achievement by providing immediate feedback and hints to students, detailed information on how students performed to teachers, and instructional suggestions for teachers to use. Researchers at the Friday Institute for Educational Innovation conducted an intrinsic, longitudinal multiple-case study of 7th-grade mathematics teachers' implementation of ASSISTments and its impact on their instruction before and during the COVID-19 pandemic. The study examined teachers' use of ASSISTments in three instructional contexts: in-person only, remote only, and both in-person and remote. Our findings indicate that teachers in all contexts changed their instructional practices for homework review and for determining whether their students had understood lessons. Teachers used the ASSISTments auto-generated reports to focus their homework reviews, based on their students' performance, and to provide instructional interventions and/or re-teaching. They also used the instructional suggestions provided by the ASSISTments platform to plan lessons to re-teach concepts or to review prior instruction with their students. [Published in "Proceedings of the 23rd International Artificial Intelligence in Education (AIED) Conference," pp.168-173, 2022.]