First of all, students must know that understanding is the simplest and most basic requirement for a student. Even a famous math teacher said that everyone can understand what the teacher is saying, and only fools can't understand it. What matters is what you do next. If you understand it in class, it doesn't mean you really will, because you think you will. This may be deceiving yourself, and your thinking is deceiving you. When the teacher is giving a lecture, you may be silently writing down the steps of that question, which is the retrospective situation, but you don't understand it yourself. You think you know some knowledge points, but there are some things you don't understand.
This requires those high school students to find the same type of questions for continuous training after the teacher finishes the questions, and ask them immediately if they don't understand, especially in mathematics. Many people reflect the problems encountered in this subject. A teacher can't do a problem that he knows how to do, not when he does it, but when he says it. This kind of learning situation is also a common state of many people. This is normal, so high school students must not worry when they encounter this situation. You should be patient. It's best to summarize and write these questions over and over again until you really understand them, instead of thinking that you really understand them.
Another situation may be that students do not have a thorough and comprehensive understanding. Sometimes, the teacher talks about a problem that requires students to do a kind of problem, even if they change it. And some students understand this problem, but they don't know how to change it. Some even changed a little, just changed a number. Some students may not know how to start. This situation requires high school students to improve their ability to deal with emergencies and analyze problems. Sometimes a problem is to analyze it and understand a method to do it, not to understand it. What makes you master is not the answer to this question, but the learning method you learned through this question.