1) You don't understand this question at all. It only makes sense when listening to the teacher. Have you thought about it after the teacher finished speaking? In this case, you don't need a long time, and one night is enough to make you forget the problem.
The solution to this situation is: the teacher should think carefully after speaking, especially what the teacher asked, what conditions he saw, and what conclusions I should think of next time I encounter such conditions. So even if you forget this question, you will remember this way of thinking.
2) Forgetting is regular, and it will be forgotten after a long time. If you want to remember, you can take a memory book and record what you think is important and easy to forget, as upstairs said. The key is to read often. It's no use just recording.
I think there is a lot of forgetfulness in the first case. After all, what the teacher said was not as thorough as he thought, so we should find ways to deepen our understanding.