Sure, let's fill in the blanks for sentences 5-10 with "will" or "be going to" and explain the reasons for each choice:

5. I don't feel like going out this evening at all. I **will** clean up my closet.
- **Reason:** The decision to clean up the closet is made at the moment of speaking, so "will" is used.

6. We are going to Sa Pa this weekend. We **are going to** take the train because it is faster than the bus.
- **Reason:** The plan to take the train is already decided and arranged, so "be going to" is appropriate.

7. There's a big traffic jam on the highway. They **will** probably go another way.
- **Reason:** This is a prediction based on the current situation, so "will" is used.

8. Do not leave your caramel saucepan unattended like that. It **will** burn pretty quickly.
- **Reason:** This is a warning about what will happen if the saucepan is left unattended, so "will" is used.

9. Do you know what to give your dad for his birthday? - I **am going to** buy him a book on gardening.
- **Reason:** The speaker has already decided to buy the book, so "be going to" is used.

10. Pete **is going to** throw a surprise party for his mother. He has talked to his father about this idea.
- **Reason:** The plan to throw the party is already decided and arranged, so "be going to" is appropriate.