Here are the completed sentences with the necessary corrections:

1. The man who paid for the meal was a friend of Tom's.

2. The meeting that we had to attend went on for three hours.

3. My office, which is on the second floor of the building, is very small.

4. Jane works for a company that makes shoes.

5. The woman who I was sitting next to talked all the time.

6. This school is only for children whose first language is English.

7. Laura had a wooden box in which she kept her photos.

8. I don't know the name of the woman to whom I spoke on the phone.

9. Towns that attract tourists are usually crowded in the summer.

10. Somewhere I've got a photo of the mountain that we climbed.

11. The sun, which is one of millions of stars in the universe, provides us with heat and light.

12. I thought I recognized the assistant who served us.

13. Which is the European country whose economy is growing the fastest?

14. John, who speaks French and Italian, works as a tour guide.

15. The detective lost sight of the man he was following. (correct without change)

16. She told me her address, which I wrote down on a piece of paper.

17. Sarah, whom you met yesterday, works in advertising.

18. I'm afraid the numbers I chose didn't win the prize. (correct without change)

19. Electronics is a subject about which I knew very little.

20. Mark, whose car had broken down, was in a very bad mood.