Please tell me if the sentences below are grammatically correct.

Please tell me if the sentences below are grammatically correct.
This is for entrance exam of Japanese Universities, so it doesn't have to be so strict and it's OK as far as they don't contain fatal grammatical mistakes.

1. My mother doesn’t put sugar when she drinks tea or coffee because she is on a diet.
2. Do you know what time the next bus leaves? Yes, it leaves at four twenty five.
3. I’ll tell you when I will arrive in Boston before I leave Japan.
4. If it rains the day after tomorrow, I’ll stay home.
5. He had to give up going to college because his father died suddenly.
6. When I dropped by that shop yesterday afternoon, they were having a bargain sale, so I bought a wallet as a gift for my mother.
7. What time is convenient for you? Around 6 o’clock in the morning, but I’ll be free all day tomorrow.
8. This task may be beyond me, but I’ll do my best.
9. I think it is around ten a.m. on November 20th that I’ll arrive in Kobe, so could you kindly come to pick me up at the station?
10. If the weather is good, why don’t we go to the park to see the cherry blossoms next Saturday?
I’m going to hold a party with some friends at my house this Sunday afternoon. Why don’t you join us?
Please don’t forget to post this letter after you get to Shinjuku.
11. You have grown so much since we met sometime ago. Is your family fine?
12. I have already read this book. Don’t you have another that I can read?
13. It has rained since this morning. A plum tree in the garden is going to bloom.
14. I’m reading a very interesting book. Why don’t you read it after I finish it?
15. As soon as I reached my classroom thinking I would be late, the class started.
16. When I came home for the first time in twenty years, I found that everything had changed.
17. The other day, when I dropped by an old book store on my way home, I happened to find the book that I had long been looking for.
18. By tomorrow, I will have finished that book which I borrowed from you.
19. I’m going to Hawaii this winter, which would be my fifth visit. [if so, I will have been there five times.]
20. Cherry blossoms will have fallen and the mountain will have been green when you get into this school.
Since we graduated from high school, I haven’t heard anything from him.
After I got on a bus, I noticed that I had left my wallet yesterday morning.