1. Steve is very tired, so he is having a nap in the room.
2. What does he do? - He is a doctor..
3. When does the play begin?
4. What are you doing..? I am playing..
5. Tina speaks two languages.
6. I am still waiting for my friend's letter.
7. What is he thinking about? (to think в значении "полагать, считать"не употребляется в continious, но в данном предложении переводится как "размышлять, думать", поэтому употребляем в continious)
8. He is taking four courses..
9. Anna is trying ...
10. I do not know his brother (to know не используется в continious)
11. Look at him! What is he doing?
12. Sam certainly is not having any fun right now.
13. Do you always look the door..?
14. Look! It is beginning to rain
15. Listen! Somebody is knocking on the door.
P.S. Указал объяснения в спорных моментах. Могу по каждому, если нужно.