Read the sentences and find the mistake and correct

II. Read the sentences and find the mistake and correct.
 1. I'm not sure about what we should to do.
2. Are we going to must work overtime again this week?
3. We have many things to do this week, so I don’t know if I can’t go to English class.
4. I have to thought about the problem for a while, but I finally figured it out.
5. You should quiet in class; otherwise, your classmates can’t concentrate.