Sure! Here are the sentences filled in:

1. The man **who** is sitting on the chair is a teacher.
2. The dog **that** is eating belongs to Mike.
3. This actor, **whose** films are very bad, is really rich.
4. People **who** the company employs are expected to sign a contract.
5. The house **that** you saw belongs to my brother.
6. The only thing **that** matters is to tell the truth.
7. That's the house **where** I was born.
8. The girl **who** phoned to my sister is her best friend.
9. Ann, **whose** children are now grown up, is looking for a job.
10. I told the police **what** happened.
11. Peter is sick. I met Peter yesterday.
12. These pupils are disruptive. These pupils **who** come into the classroom are disruptive.
13. My car is broken. I crashed my car in a roundabout.
14. That is my bus. I usually take that bus.
15. Here is a game. I don't understand the rules of the game. Here is a game **that** I don't understand.
16. John is fat. The appetite of John **who** is spectacular, is fat.
17. My book has been stained. I put my book into the trash bag.
18. My father is coming back. My father **who** is coming back is in a bad mood.
19. Maggie is crying. I comfort Maggie **who** is crying.