Here are the sentences completed with the correct verb forms:

1. Are you interested in **doing** volunteer work?
- Yes, I **joined** my school volunteer team last summer.

2. Please sit down and take a rest. I **will finish** my work soon before I join the game.

3. Unless you **understand**, I will explain it again to you.

4. Let’s **participate** in the “Great Summer” programme, it’s really great one.

5. Jane’s hobby is **cooking**. She’s keen on **preparing** food for her family and friends.

6. We find **arranging** flowers interesting because it **helps** us relax.

7. He says that he will **arrive** on time tomorrow.

8. If he **is** a student, he will get a discount.

9. It was at the age of 14 that Jack London **left** school and **became** a sailor.

10. When he lived in Manchester, he **worked** in a bank.

11. Mr. Smith **taught** at this school after he **graduated** in 1990.