Here are the sentences combined into single sentences using relative clauses:
1. The first boy who has just moved knows the truth.
2. I don't like the man who is going out with my sister.
3. Yesterday I met a man who works with your brother.
4. I know a shop that sells really good meat.
5. I watched a movie last night that was interesting.
6. The boy whom I know from school is very nice.
7. The shoes that my mother bought for me are too big.
8. Kate helps a man who is my teacher.
9. I still have the book that you gave me.
10. Did you hear about the earthquake that occurred in California?
11. A river that is polluted is not safe for swimming.
12. The doctor who examined the sick child was very gentle.
13. The man whom I was telling you about is standing over there.
14. The meeting that I went to was interesting.
15. The student who studies Vietnamese is from China.
16. People who live in Scotland are called Scots.
17. He is reading a book that you lent me yesterday.
18. Bring me the book that I left on the table by the window.
19. Jane, whom I have known for eight years, is one of my closest friends.
20. This is the book that I like very much.