Answer:
Let's correct the sentences:
1. "I think she must be very rich."
2. "They want us to leave by tomorrow. You mustn't have to be serious."
3. "I heard about Jane's accident. You must have been worried about her."
4. "He says he's 50, but he hasn't got to be that old."
5. "I looked at my watch and didn't have much time."
6. "To get to Peru, I had to borrow money from time to time."
7. "Being so well-known, you must receive hundreds of letters each week."
8. "Can we meet on Thursday morning?' 'Sorry, no. I have to go to the dentist at 11.00."
9. "I'm feeling really unfit. I have to do more exercise."
10. "He's got a lung problem and he has to go to the hospital every two weeks."