Generally, there is more than one topic in a class. Don't dwell on one topic, make a mark and move on to the next. But if you don't know, ask the teacher immediately after class. Learning focuses on self-study, self-experience and practice.

Especially in the subject of mathematics, it is very necessary to do problems. You can sum up the rules in practice and cultivate your interest in doing problems. If you can't keep up, you can preview it before class, and you won't ask your teachers or classmates after class.