Answer:

1. Books are interesting.
2. White mice are in the box.
3. His sisters are secretary.
4. They are my favourite actor.
5. Rivers were dirty.
6. My friends are students.
7. Bookshelves are between the mirror and the sofa.
8. My children are pupils.
9. Her dresses are beautiful.
10. Our puppies are hungry.
11. These pages were yellow.
12. Men are brave.
13. Grasses are empty.
14. My pupils are very clever.
15. Our rooms were comfortable.