Here are the correct forms of the verbs to complete the sentences:

1. They (not buy) **haven't bought** a house yet.
2. Meg and Steve (be) **have been** to Japan once.
3. I (see) **saw** a volcano. It’s exciting.
4. We (play) **play** different sports at school.
5. The children (finish) **have finished** their homework, so now they are playing football in the park.
6. Mary (watch) **has watched** this film many times on TV.
7. Mary (not write) **hasn't written** the email to her best friend.
8. I and my friend (not finish) **haven't finished** this exercise.
9. You (make) **made** the cake?
10. My brother (never see) **has never seen** this film.
11. You (ever see) **have you ever seen** this film?
12. Our school team (just win) **has just won** the competition.
13. They (already write) **have already written** the test in Maths.
14. He (not prepare) **hasn't prepared** the report for the conference yet.
15. I (already receive) **have already received** an invitation to the wedding.
16. She (never eat) **has never eaten** sushi in Japan.
17. We (not meet) **haven't met** at conference yet.
18. We (just miss) **have just missed** the bus.
19. They (already do) **have already done** their homework.
20. What you (already buy) **have you already bought** for her birthday?

Feel free to ask if you need further assistance!