1.This is a book.–This isn't a book.

2.I can sing.–I can'tsing.

4.He was at home.–He wasn't at home.

5.I'll go tomorrow.–I won't go tomorrow.

7.I have been to Beijing.–I haven't been to Beijing.

8.My sister could speak English two years ago.–My sister couldn't speak English two years ago.

9.Lisa does her homework in the evening.–Lisa doesn't do her homework in the evening.