'Have to' and 'must' are used to express strong obligation. There is a slight difference in the meaning although people often use them in the same way. We use 'must' when the obligation comes from personal circumstances and the speaker agrees with the obligation. 'I must go to bed earlier.' We use 'have to' when the obligation comes from external circumstances or an external agent such as our jobs. 'I have to be at work by 9am. My boss is very strict about it.' In British English we also use 'have got to' in the same way as 'have to'. In the negative the two verbs are quite different in meaning. 'Don't have to' is used to say something is not necessary. 'You don't have to come, if you don't want to.' 'Mustn't' is used to express a strong obligation not to do something. 'You mustn't phone me at work. I'm not allowed personal calls.' 'Must is also used to make deductions or show that we are certain something is true. 'It must be nice to live in Florida.'

"Have to" , "has to" and "must" have the same meaning in the affirmative and interrogative forms when referring to obligation. Some grammarians think that "must" is slightly stronger, but for all practical purposes, they mean the same thing: Example: A doctor has to/Doctors have to/ attend medical school for several years before he/they can practice medicine. Doctors must attend medical school for several years before they can practice medicine. While "have/has to" and "must" can be used interchangeably, there are differences in usage: Both verbs can be used in British English to talk about obligation. (In American English, 'have to' is the normal form.) British English often makes a distinction as follows. 'Must' is used mostly to talk about the feelings and wishes of the speaker and hearer — for example, to give or ask for orders. 'Have (got) to' is used mostly to talk about obligations that come from "outside" — for example from laws, regulations, agreements and other people's orders. Compare: I must stop smoking. (I want to.) I have to/she has to stop smoking. Doctor's orders. This is a terrible party. We really must go home. This is a lovely party, but we've got to go home because of the baby-sitter. Must you wear dirty old jeans all the time? (Is it personally important for you?) Do you have to/does he has to wear a tie at work? (Is there a regulation?)

It's really pretty compex, come to think of it, much more to this. I hope English speaking comrades feel it better, and can explain it in great detail.

Must is when the others want you to, obligation. Have to is when you make the decision yourself. Have to - or got to. I gotta see her. Something like this.