Sure! Here are the appropriate choices for each sentence:

1. If they spend more time reading materials, they **might** understand the topic more. (could, might, and may are all possible, but "might" suggests a weaker possibility, which is appropriate here)

2. Minh **can** help you with your homework, if you ask him nicely. (This indicates that it's possible for Minh to help)

3. If you join in cooking courses, you **will** prepare your own food. (This indicates that it's a certain outcome)

4. If you have many assignments at the end of this semester, you **should** go to the self-study room. (This suggests a recommendation)

5. Teenagers **can** help you with your homework, if you ask him nicely. (This indicates possibility)

So the answers are:
1. might
2. can
3. will
4. should
5. can