Please check for grammar and sentence structure. Thanks!

1. Most of the students are wonderful; sometimes I’m amazed how they’re so motivated to learn.
2. Now I’m really impressed by whichever  what she’s learned.
3. Sometimes we spend all afternoon on one thing, but that’s OK; what Susan's so motivated Susan's motivation (OR Susan being so motivated) is very rewarding.
4. How do we decide who wins the prize?
5. Mr. Brown, I’m sorry to report that I need more time to complete the assignment.
6. As for the problem of stopping violence in schools, I don’t know which what the answer is.
7. We haven’t decided where we want to go on vacation.
8. An extrovert is basically a person whose energies are activated by being with others.
9. Good examples are Jack and Barbara, who have been married for years.
10. The loan officer, who makes Gary feel criticized, is only trying to do this? job. (should be "his" or "her" job)
11. The members of my family, who are all interesting, fit nicely into the Meyers-Briggs personality categories.
12. Anyone who is interested in drama should see this play.
13. The neighborhood whose?  (this should indicated who is moving into the neighborhood. Is it "we", "they"?) are moving into is much safer.
14. Whatever (whenever✅ however✅ (If you use however, it means "With whatever method I use to exercise, I feel good" ) I exercise, I feel good.
15. I have to practice a lot because I’m on the team.
16. The race will start when everyone is in place.
17. Because of his constant practice, he won’t make a team. This sentence is grammatically correct, but it is seems rather conflicting. He should make a team if he constantly practices right?
18. As you train a great deal, you won’t be a champion. Same as above, grammatically correct, but seems conflicting.
19. Since that fixing the flat tire, we were on our way again.
20. Because I was given two options., I chose the harder of the two.

If you are trying to use "since" in the temporal sense, you should be saying something like: - I haven't heard from her since she left London. (or, moved away from London) - I haven't heard from her since she moved to London. - I have been in poor health since I started smoking cigarettes. "I have been in poor health since I began to smoke." This means that you have been in poor health BECAUSE you began smoking. If you replace "since" with "because" you can probably understand it better: "I have been in poor health because I began to smoke." If you want to say that you have been in poor health from the time you started smoking, you should say something like: "I have been in poor health ever since I started smoking"

Winter Vacation

When winter comes, I would like to  take a quick holiday. The weather bureau says that it will be very cold in this year, but in Guangdong province, it never snows. I want to see snow so I am planning to going to see the snow.

Notes:

1. Instead of "The weather bureau says", you could say "The weather bureau forecasts".

2. You can optionally leave out "province" after Guangdong.