Mark explains the use of have to to describe outside obligations.

We use HAVE TO to talk about obligations:

- I have to finish my homework before I go to bed
- You have to be 18 years old to buy a house

We can also say HAVE GOT TO or GOT TO:

- I have a car --> I have to get up early
- I've got a car --> I've got to get up early [British]
- I got a car --> I got to get up early [American]
- (I gotta car --> I gotta get up early) [American colloquial]

- I don't have a car --> I don't have to get up early
- Do you have a car? --> Do you have to get up early?
etc.

MEANING

The affirmative form expresses obligation:

- You have to get up at 8 (you have no choice, you must do it)

The negative form expresses absence of obligation (no obligation):

- You don't have to get up at 8 (but if you want, you can)

We only use HAVE TO for external obligation. Compare

- You have to be quiet in a hospital (those are the regulations)
- You must be quiet (I tell you so)
- You can't speak in a hospital (prohibition)