Must and have to

A key distinction between must and have to can be found in the negative forms. Whereas ‘You must go’ and ‘You have to go’ can be regarded as broadly the same in terms of meaning, ‘You mustn’t go’ and ‘You don’t have to go’ are quite different, the first indicating that going is prohibited in some way, or even dangerous, while the second implies an absence of obligation or need.

Another difference between must and have to in the sense of obligation can be found in the nature of the obligation. It is possible to say ‘I’m sorry. I can’t come to the meeting tomorrow because I have to go to the dentist at 3 o’clock’ but not ‘I can’t come to the meeting tomorrow because I must go the dentist at 3 o’clock’. On the other hand, if you have a raging toothache, you would probably say ‘I really must go to the dentist’, although have to could replace must in this sentence. A generalized distinction would be that must refers to an internal need or obligation while have to is used to refer to an external need or obligation. It is probably true to say, however, that must can generally be replaced by have to but have to often cannot be replaced by must. Must and must not are useful for official notices and instructions, e.g. ‘You must carry your passport at all times’ and ‘You must not smoke in the toilets’.

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to have (to) / to have got (to) [hxv] в модальном значении выражает необходимость совершения действия, вызванную обстоятельствами – должен, придется, вынужден. По смыслу близок модальному must (обязанность или необходимость с точки зрения говорящего). В этом значении он может употребляться во всех формах и временах, в предложениях любых типов в сочетании с простым, неперфектным инфинитивом (Indefinite Infinitive) с частицей to. Он имеет формы времени: have / has – настоящее время, had – прошедшее время, shall / will have – будущее время.