15. had never seen - visited
- QKHT before QKĐ
16. am doing
- S am/are/is Ving
17. am reading
- S am/are/is Ving
18. am still waiting
- S am/are/is Ving
19. are they speaking
- Wh- am/are/is S Ving?
20. is stealing
- S am/are/is Ving
21. was working - was shopping
- S was/were Ving