Exercise 'Might'

Hello!
Please correct me! These is are exercises from Murphy's book.

1. Helen might be busy.
2. She might be working.
3. She might want to be alone.
4. She might have been ill yesterday.
5. She might have gone home early.
6. She might have been working yesterday.
7. She might not want to see me.
8. She might not be working today.
9. She haven't been felt well yesterday.
10. Who is that man with Emily? I'm not sure. It might be her brother.
11. Who was the man we saw with Anna yesterday? I'm not sure. It may have been her brother.
12. What are those people doing by the side of the road? I don't know. They might be waiting for a bus.
13. Do you have a stamp? No, but ask Simon. He may has have [This is because of the word "may", a modal form.  This is why learning English is so difficult] one.
14. I'm looking for Sarah. Do you know where she is?
She might be watching TV in her room.
She might have gone out.
15. I can't find my umbrella. Have you seen it?
It might be in the car.
You might have left it in the restaurant last night.
16. Why didn't Dave answer the doorbell? I'm sure he was at home at the time.
He might have gone to the bed early.
He might not have heard the doorbell.
He might have been in the shower.
17. - I wonder why they never replied to our letter. Do you think they received it?
- Maybe not. They might not have received it?
18. - I wonder how the fire started. Was it an accident?
- No, the police say it couldn't have been the an accident.
19. - Mike says he needs to see you. He tried to find you yesterday.
- Well, he might not have been looking for me very hard. I was in my office all day.
20. - The man you spoke to - are you sure he was American?
- No, I'm not sure. He couldn't be American.

Thank you for your help!