Must
Obligation: People must abide by the law. (if they don �t, they get penalized for that).
Certainty: Jimmy gets the worst grades in class. He must be negligent.
Necessity: It �s getting late. I must go home now. (British English)
Deduction (certainly): Your eyes are red and swollen. You must have cried a lot.
Have to
Obligation: My parents say i have to tidy up my room before I leave to school. (Obligation
imposed by the speaker)
Necessity: I �m putting on weight. I have to go on a diet. (US English)
Mustn �t:
Prohibition: Students mustn �t cheat in exams.
Do(es)n �t have to = do(es)n �t need to
Lack of necessity: You don �t have to make a sandwich for me, mum. I �m not hungry.