Set and subset are very simple concepts. If you can't do the problem, you don't know enough about this concept. Look at the concept several times, circle the key points and scrutinize them carefully. Of course, if the topic itself is difficult, it is another matter. Since it is a rehearsal, it is good to understand it. There is no need to ask too much. In addition, I don't know if you know the essence of sets, such as certainty, mutual difference and disorder, because your question is not specific and it is really difficult to answer. If you really want to study deeply, I suggest you buy a copy of Wang Houxiong Learning Case, which is very useful. I used it in high school for three years.