This article highlights the importance of diagnosing students' knowledge in the effective teaching of mathematics at the primary education level. Through the diagnostic process, the students' knowledge level, individual characteristics, and learning approaches, in particular, the effectiveness of using interactive methods, tests, practical tasks, and the EGMA evaluation program were analyzed. Also, the role of formative assessment in monitoring the dynamics of student development is revealed. According to the results of the research, it was concluded that regular and systematic assessment serves to deepen students' mathematical knowledge, develop independent thinking and problem-solving skills.