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.

Must, have to, should (obligation)

obligation / necessity: have to / must (infinitive)
Must and have to have a very similar meaning.
Have to is more common for general, external obligations, for example rules and laws.
Must is more common for specific or personal obligations.
Compare: I have to wear a shirt and tie at work (It's the rule in this company). I must buy a new shirt - this one is too old now (It's my own decision).
Have to is a normal verb and it exists in all tenses.
Must is a modal verb. The only forms are must and mustn't.
You can also use have to or must for strong recommendations. You have to/must see that film. It's fantastic.

no obligation / necessity: don't have to. ---- prohibition: mustn't (+ infinitive)
Don't have to and mustn't are completely different.
Compare: You mustn't drive along this street (It's prohibited, against the law). You don't have to drive - We can get a train (You can drive if you want to but it's not necessary/obligatory).
You can often use can't or not allowed to instead of mustn't.

advice or opinion: should/shouldn't (+infinitive)
Should is not as strong as must/have to. We use it to say if we think something is the right or wrong thing to do.
Should is a modal verb. The only form are should or shouldn't.
You can also use ought to and ought not to instead of should/shouldn't.

Should / Shouldn't (advice)

Use should / shouldn't + infinitive to give somebody advice (consejo) or say what you think is the right think to do.
You should cut your hair. = I think it would be a good idea.

Should / shouldn't + infinitive is the same for all persons.

You can use ought to / ought not to (es más formal) instead of should / shouldn't.