Have to

See main article Have. Have (got) to is one of the most frequent structures used to express obligation, especially when coming from “outside”, imposed by an authority, reality, etc.

Devoicing
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/have got to
Have/have got (has/has 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.).

Future
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.

Gotta
Notice also the non-standard gotta.