Reason one: complete thinking. It is best to read the introduction and derivation of the theorem several times, with the emphasis on understanding. If you can't remember, you can always turn over the books.

Reason two: it's just part of the idea. Explain that you don't fully understand all kinds of classic questions, do more examples and after-school questions, and write down what you can't do in your notebook.

Reason three: thinking but miscalculating. Do more problems and don't be afraid of trouble. Trouble is better than nothing.