Here are the correct forms of the verbs for each sentence:

1. She is sad because her son **doesn't visit** her very often.
2. Hoa (come) **came** from Hue, but now she (live) **lives** with her uncle in Ha Noi.
3. I’m busy at the moment. I **am decorating** the living room.
4. We (live) **have lived** here since I **was** ten.
5. Because Minh (never work) **has never worked** for a charity organisation, he really (want) **wants** to do it.
6. She (be) **has been** to Sa Pa many times.
7. Jane (work) **has worked** for this company for 6 years.
8. Do you hear what she (just say) **just said**?
9. Lan (move) **moved** to Ho Chi Minh two years ago and I (not meet) **haven't met** her since then.
10. Peter (buy) **bought** a new CD player yesterday, but I (not see) **haven't seen** it yet.
11. Frank did his homework, then (listen) **listened** to music.
12. I (not study) **didn't study** hard last year.
13. Where (you be) **were you** last night? I (phone) **phoned** you but no one (answer) **answered** me.
14. When he was young, he (can speak) **could speak** English well.
15. Next week we (have) **will have** many kinds of examinations. It (be) **will be** a very busy week, I think.
16. He (phone) **will phone** us before he leaves home tomorrow.
17. I hope I (be) **will be** on time for the meeting.
18. I (see) **will see** John at 8 o'clock tonight.
19. Students are interested in (play) **playing** computer games.
20. I’m tired of (have) **having** nothing to do at home.
21. How about (go) **going** to the zoo this weekend?
22. He left without (take) **taking** his coat.
23. Every girl should (learn) **learn** how (cook) **to cook**.
24. I prefer (walk) **to walk** to (ride) **riding** in the garden when it is fine.
25. Lan enjoys (sit) **sitting** in the garden when it is fine.

Feel free to ask if you need further assistance!