Obligation and necessity

Must/mustn't + infinitive without to

Must is used:
for strong obligations imposed by the speaker. The speaker uses must to express his/her authority. You must be here by 8 am.
to give strong advice. It's a great film. You really must go and see it.
to tell oneself what is necessary. I must remember to phone Tim.

Must not or mustn't is used:
to talk about something that is not permitted. Passengers must not smoke on the aircraft.
to give strong advice. You mustn't work too hard. You'll make yourself ill.

Past form
Must does not have past form. Had to is therefore used to refer to the past. We had to write a formal letter in the exam. 

Question form
Must is possible in question forms. Must you wear that horrible dress?
although have to is more common. What do we have to do for homework?


Have to/Don't have to

Have to is used to refer to strong obligations imposed by another person rather than by the speaker or writer. I have to be at work by 8 o'clock. The boss will get angry if I'm late.


Need to/Don't need to/Needn't 

Need to is used to express necessity. Can we go to the baker's next? I need to get some bread.


Should/Shouldn't + infinitive without to

Should and shouldn't are used to express obligation or give advice. Ought to can also be used with the same meaning as should. You ought to/should see a doctor about your headache.


Be supposed to/Had better

Be supposed to is used to talk about what you should do because of a rule or because it is expected. Come on, it's 10 o'clock. You're supposed to be in bed!

Had better + infinitive without to is used to talk about what you should do because you think it's a good idea. You'd better ask your dad before you borrow the car.