1. The person(man) whom you met yesterday is my high school friend.
2. He is the ideal man(guy) whom I want to (would like to) date.
3. She is my colleague (co-worker) who helps me with my project (who is helping me with the project).
4. The teacher who gave me one of my lessons last year passed away.
-> The teacher who taught one of my classes last year passed away.
5. The picture which he bought is hung on his living room.
-> The painting that he bought hangs in his living room.
6. I lent him the camera which my father bought to me.
7. The broken machine will be fixed tomorrow. -> The machine that is broken
8. He recommended me a book which was on the best-seller list is in the list of best seller.
9. My grades went down, which makes my parents worry.
-> My grades fell, which worried my parents.
10. My favorite childhood memories are the times when I played the baseball with my father.
11. I know the a girl whose father is a professor at this college.
12. I cannot don't understand the way she thinks.
13. There is no reason you can't don't go.
14. The department store where we shopped was packed with people.
15. The students who are preparing in advance usually do well on tests.
-> The students that prepare in advance~