- About "have to", "has to", "don't have to" and "doesn't have to".
- About "must" and "mustn't".
- What is the difference between must and have to?
- What is the difference between mustn't and don't have to?
| Usage | Examples |
| In the affirmative have to / must both express obligation. "It is necessary to do something." | I have to leave now. He has to go now. I must leave now. He must go now. |
| In the affirmative have to expresses impersonal obligation. There is a rule or an external power. must expresses personal obligation. Must expresses what the speaker thinks is necessary. | Students have to wear school uniforms. She has to wear her school uniform. You have to be at work at nine. I must wear a stylish jacket tonight. He must study harder. |
| In the negative doesn't / don't have to means "It is not necessary to do something." | You don't have to be smart to be rich. I don't have to get up early tomorrow. She doesn't have to talk to me. |
| In the negative mustn't expresses prohibition: - Don't do it! | I mustn't watch 4 hours of TV a day. personal You mustn't park your car here. impersonal |
