Here are the sentences with the mistakes underlined and corrected:

1. When **is** he **doing**?
**Correction**: When **is** he **going**?

2. What is Anna doing? - **He** is reading books.
**Correction**: What is Anna doing? - **She** is reading books.

3. What are Mai and Hoa **do**?
**Correction**: What are Mai and Hoa **doing**?

4. They're skating **from** the skate park.
**Correction**: They're skating **at** the skate park.

5. What are Linh and Hoa doing? **She** is singing at the studio.
**Correction**: What are Linh and Hoa doing? **They** are singing at the studio.

6. What is **Peter's** doing?
**Correction**: What is **Peter** doing?

7. They're **play** video game at the arcade.
**Correction**: They're **playing** video games at the arcade.

8. **She's is** flying a kite at the park.
**Correction**: **She is** flying a kite at the park.

9. The children **a** playing volleyball.
**Correction**: The children **are** playing volleyball.

10. I'm playing soccer at the soccer park.
**Correction**: (No correction needed; this sentence is correct.)