use 'must' to express a strong obligation. When we use 'must' this usually means that some personal circumstance makes the obligation necessary (and the speaker almost certainly agrees with the obligation.) i must study hard to get the full mark. i must go to bed early. use 'have to' to express a strong obligation. When we use 'have to' this usually means that some external circumstance makes the obligation necessary. i have to go to work at 9 or the boss will be angry

In affirmative statements the meanings are the same. In North American English, "have to" is much more common, especially in speaking. "I have to work a lot this week." "Tom has to speak to someone at registration to get into the class." "Our team has to win this game to get into the finals." You can find "must" in official writings, such as rules and regulations. "The applicant must bring a photo ID to the interview." In the negative, they are very different. "You must not talk during the exam." (That is the rule. Don't do it!) "You don't have to know English to come to the US." (There is no requirement.)

Sometimes we can use must or have to to mean the same thing. Example: I need to be at the airport in one hour, so I must / have to go. Generally, we use must for expressing personal feelings, and have to for expressing facts. Example: That new restaurant is great! You must go there! You have to turn off your mobile phone in a museum. We use must for the present or future, but not the past. We can use have to for all tenses (i.e., change have to to had to). For negative statements and questions, we generally use do / does / did with have to. Example: You don't have to participate in the phone conference. CORRECT You have not to participate in the phone conference. INCORRECT Why did you have to tell the boss what I said? CORRECT Why had you to tell the boss what I said? INCORRECT Must not / mustn't and don't have to have different meanings. Must not / mustn't means it is forbidden. Don't have to means it is not necessary. Example: You mustn't do personal work on the company's computers. You don't have to attend the meeting if you don't want to.

In most situations, must=have to. However in certain situations, we use the word must instead. eg. must not... or this is a must.

We can also use ' will have to' to talk about strong obligations. Like 'must' this usually means that that some personal circumstance makes the obligation necessary. (Remember that 'will' is often used to show 'willingness'.) I'll have to speak to him. We'll have to have lunch and catch up on all the gossip. They'll have to do something about it. I'll have to get back to you on that. As you can see, the differences between the present forms are sometimes very small and very subtle. However, there is a huge difference in the negative forms. We use 'mustn't' to express strong obligations NOT to do something. We mustn't talk about it. It's confidential. 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. We use 'don't have to' (or 'haven't got to' in British English) to state that there is NO obligation or necessity. We don't have to get there on time. The boss is away today. I don't have to listen to this. I'm leaving. You don't have to come if you don't want to. He doesn't have to sign anything if he doesn't want to at this stage. I haven't got to go. Only if I want to