Sure! Here are the rewritten sentences based on your prompts:

9. My brother prefers watching cartoons over listening to music.
10. They have known each other for 3 months.
11. Nam’s teacher has been teaching him since he came here.
12. We haven’t gone on a picnic since the start of Covid-19.
13. Which place have you been to since you left school?
14. Yesterday, Peter answered the questions more quickly than anyone in my class.
15. He studies hard, so he gets good marks.
16. She is sad because she received a bad mark yesterday.
17. Linh did not go to school last Wednesday because she had a bad cold.
18. She failed her English exam last week because she did not study hard.
19. She practices playing volleyball every day, so she can do it well.
20. Why were you late for school yesterday? My bike broke down.
21. Why didn’t you finish your homework? I have had a bad cold for three days.