18. ✓
17. The man asked the boys whether they knew where his son was
18. The boy asked his mother whether his mother was going to give him a present the following day.
19. ✓
20. ✓
21. ✓
22. She asked us if we had to go then.
- must, should, ought to,... `->` had to+Vo (lùi thì)
23. Paul asked his sister if he had to put that wardrobe there.