:

1. When I **was** (past simple of "be").
2. I **lived** (past simple of "live") as a child. I **used** (past simple of "use") to...
3. Up to now, the teacher **has given** (present perfect of "give") us five tests.
4. She **has done** (present perfect of "do") her homework recently.
5. Lan **has seen** (present perfect of "see") this film three times.
6. We **have been waiting** (present perfect continuous of "wait") for you since two o'clock. We **have to study** (present simple of "have to study") hard.
7. When you **started** (past simple of "start") school? When I **was** (past simple of "be") six.
8. Nam **bought** (past simple of "buy") a new dictionary yesterday.
9. His hair is very short. He **has had** (present perfect of "have") a haircut?
10. Last night my father **arrived** (past simple of "arrive") home late. He **had** (past simple of "have") a bath and then **went** (past simple of "go") to bed.
11. The floor looks dirty. I'm sure you **didn't clean up** (past simple negative of "clean up").
12. Did you **enjoy** (past simple of "enjoy") the party last night?
13. My bicycle isn't here. I think someone just **took** (past simple of "take") it.
14. They **have known** (present perfect of "know") each other since they **were** (past simple of "be") children.
15. They are the most kind-hearted people he ever **met** (past simple of "meet").
16. He **was** (past simple of "be") late yesterday because he **was** (past simple of "be") stuck in the traffic jam.
17. Last year, he **donated** (past simple of "donate") 99% of his fortune to charitable organizations.
18. We **haven't visited** (present perfect negative of "visit") our hometown so far.
19. Would you mind if I **sat** (past simple of "sit") here?
20. I **have been** (present perfect of "be") a member of this non-profit organization since 1996.

3.has given
4.has done
5.has seen
6.have waited
7.đi you start - was
8.Did Nam buy
9.Has he had
10.arrived / had/went.