1.1. I'm the best in the class. → I'm not the best in the class.

2.2. Mark does do homework in the evening. → Mark doesn't do homework in the evening.

3.3. We play football after school. → We don't play football after school.

4.4. You run fast. → You don't run fast.

5.5. Tim and Lucas  work at weekends. → Tim and Lucas don't work at weekends.

6.6. I like classical music. → I don't like classical music.

7.7. These exercises aen difficult. → These exercises aren't difficult. 

8.8. She has science on Fridays. → She doesn't have science on Fridays.