It looks like you have a list of sentences that need to be rewritten using comparative structures. Here are some corrections and improvements for each line where applicable:

1. **John can walk farther than I can.** (Correct)
2. **I can’t walk as far as John.** (Correct)
3. **Tom can play tennis better than his brother.** (Correct)
4. **Tom's brother can't play tennis as well as Tom.**
5. **No other student in this class is as tall as Tom.** (Correct)
6. **Tom is taller than any other student in this class.**
7. **Among the students in this class, Jane is the best one.** (Correct)
8. **No other student in this class is better than Jane.**
9. **Students become more nervous as they get closer to their exams.** (Correct)
10. **The closer they get to their exams, the more nervous students become.**
11. **Dogs become less aggressive as they get older.** (Correct)
12. **The older dogs get, the less aggressive they become.**
13. **We discussed this matter more. However, it became more complicated.** (Correct)
14. **As I carried the shopping bags longer, they became heavier.** (Correct)
15. **If I stay longer at home, I feel like moving less.** (Correct)
16. **I buy more and more books; it then becomes more difficult to choose what to read.**
17. **You will become more fluent if you write more.** (Correct)
18. **People become more tolerant as they grow older.** (Correct)
19. **If I spend less, I’ll save more money.**
20. **I have more and more exams these days; therefore, I get more tired and sleepless.**

If you have specific requirements for changing any of the sentences or need further assistance, let me know!