Modals are often used to express obligation, or the need to act. We use these:
HAVE TO/DON'T HAVE TO: obligation from someone else (law, rule)
2. She has to tell her boss, or she will lose her job.
3. I'm afraid I have to refer you to a general practitioner.
MUST/MUSTN'T: obligation from the speaker (neither a law nor a rule)
1. I must remember to get a present for my girlfriend.
2. We must go home now, it’s getting late.
3. What? You must be crazy, man!
NEED TO/NEEDN'T: express something less urgent, something in which you have a choice.
1. You needn’t take an umbrella. The rain has just stopped.
2. Everyone needs to make an appointment to see the doctor before next week.
3. You needn't undress down to your underwear for the consultation.
SHOULD/SHOULDN'T: gives a recommendation, and sometimes an opinion.
1. I should stop smoking. Cigarettes are too expensive these days.
2. You shouldn’t forget to ask the coach about the match.
3. Tell him I said he should walk briskly for 30 minutes every day.
NB: It is important to realise that the negatives of “have to” and “must” are different. They are “don’t have to” and “mustn’t.”
DON’T HAVE TO means that you are under no obligation to do something.
MUSTN’T means that doing something or taking action about something is forbidden!
1. You don’t have to do the grammar exercise / Vous n'êtes pas obligé de faire l'exercice de grammaire
2. You mustn’t do the grammar exercise / Vous ne devez pas faire l'exercice de grammaire
3. As you can see, they are quite different, unlike their affirmative counterparts, which are different.