Sure, here are the correct forms of the verbs in the sentences:

21. I just **lost** (lose) my keys, so I can't open that door.
22. Columbus **arrived** (arrive) in the New World in 1492.
23. Nina just **broke** (break) her leg. She is still in hospital.
24. Last winter, Hoa **stayed** (stay) with her sister in Sapa for three days.
25. **Has** she ever **gone** (go) to Bac Ninh?

Exercise 5:
1. I **have seen** (see) her before.
2. I **saw** (see) her last year.
3. Tom **has never been** (never be) in Hanoi.

The underlined parts are the signals for the correct tense usage.