Here are the correct forms for the sentences you provided:

21. Nam and Mai **are doing** the housework now.
22. What **is** your mother **doing**?
23. How **do** you **go** to school every day?
24. Nam **goes** to work by car every day, but today he **is taking** a bus.
25. Where **does** your father **work**?
26. Look! The teacher **is coming**.
27. She is very clever. She **speaks** a language.
28. Steve often **smokes** 10 cigarettes a day.
29. We usually **have** dinner at 7:00.
30. Anne **washes** her hair twice a week.
31. Jane **teaches** English to children.

Let me know if you need further help!