Have to (/hæf tu:/) is an English semi-modal verb. It is one of the most frequent structures used to express obligation, especially when coming from “outside”, imposed by an authority, reality, etc. It uses the verb have with a to-infinitive as the main verb.
Normally, we use have to to express a future event that we see as an obligation now. If we refer to a more distant future, we can use will have to, as in We'll have to do it sooner or later.
The negative is "don't have to" - and if you don't have to do something you can do so if you want to, but are not obliged. Note this is different from mustn't.
Have got to is very often used interchangeably with have to but actually expresses more immediate obligation: I have to do it sometime today. vs I've got to do it right now. We do not use "got" in the past tense or "pure" future, (I had to do it - I'll have to do it later.), except when using the present tense as a future (I have to do it later.).
This conjugates with 3rd person -s in the present tense. The past tense form and past participle form is "had to". The present participle form is "having to".
Normally there is devoicing /'hæf tə, 'hæs tə, 'hæt tə/ in spoken sentences: I have to go; He has to go; I had to go.
Have to is slightly more formal than the related "have got to"
The grammar for "have to" is the same as the grammar for "need to".
- John C. Wells. close, comment of 27 August 2012 at 20:21.