Have to do - Responsibilities
Use 'have to' in the past, present and future to express responsibility or necessity. NOTE: 'have to' is conjugated as a regular verb and therefore requires an auxiliary verb in the question form or negative.
- I have to arrive at work at 9 sharp. My boss is very strict.
- We have to give him our answer today or lose out on the contract.
- You have to pass your exams or the university will not accept you.
- I have to send a report to Head Office every week.
Must do - Obligations
Use 'must' to express something that you or a person feels is necessary. This form is used only in the present and future.
- I must go to bed earlier.
- They must do something about it.
- You must come and see us some time.
- I must say, I don't think you were very nice to him.
Don't have to do - Not Required
The negative form of 'have to' expresses the idea that something is not required. It is however, possible if so desired.
- I don't have to listen to this. I'm leaving.
- You don't have to come if you don't want to.
Mustn't do - Prohibition
The negative form of 'must' expresses the idea that something is prohibited - this form is very different in meaning than the negative of 'have to'!
- I mustn't eat chocolate. It's bad for me.
- You mustn't phone me at work. We aren't allowed personal calls.
- They mustn't see us talking or they'll suspect something.
Structure of Have to
Structure of Must