Have to, Must

 

Have to / Don't have to
Positive	She has to get up at 8.00 every week.
	You have to drive on the right in Spain.
Negative	We don't have wear a uniform at this University.
	He doesn't have to work on Mondays.
Question	Do I have to buy a dictionary?
	Does he have to study tomorrow?
Don't contract have or has !!!!

Use have to + infinitive to talk about rules and obligations, or to say something is necessary.
Use don't have to + infinitive to say there is no obligation, or something is not necessary.
Use do/does to make questions and negatives.

 

Must / Mustn't
Positive	You must do your homework tonight.
	She must tidy her room before she goes out.
Negative	You mustn't drink in class.
	They mustn't leave their bags there.

Use must + infinitive to talk about rules and obligations:
You must turn off your mobile phones before coming into class.
Must/mustn't are the same for all the persons. It is not often used in questions.
Use musn't + infinitive to say something is prohibited.
You mustn't smoke here.
You can use mustn't or can't to talk about rules.
You can't park here.

!!!!
- The small difference between Must and Have is that we normally use have to for a general obligation (a rule at work or law). We normally use must when the speaker imposes the obligation (for example the teacher to students or parents to childrens).

- Mustn't and Have to have completely different meanings.
You mustn't go = You can't go, it's prohibited.
You don't have to go = You can go if you want, but it's not obligatory/necessary.

Y aquí unos ejercicios para practicar:
http://www.englishclub.com/grammar/verbs-modals-have-to-must-not-quiz.htm