Modal Verbs of Obligation

We can use have to + infinitive, must + infinitive and should + infinitive to express obligation (something you have to do).

Present	Positive	Negative
have to / don't have to	strong obligation (possibly from outside) Children have to go to school. (sometimes 'have got to')	no obligation I don't have to work on Sundays. You don't have to eat anything you don't like.
must / mustn't	strong obligation (possibly based on the speaker's opinion) I must study today.	negative obligation You mustn't smoke here.
should / shouldn't	mild obligation or advice You should save some money.	mild negative obligation or advice You shouldn't smoke so much.

Be careful about the difference between mustn't and don't have to!
Mustn't means it's not allowed, or it's a bad idea:

Don't have to means you don't need to do something, but it's fine if you want to do it:

I don't have to get up early at the weekend(of course, if I want to get up early, that's fine, but I can stay in bed if I want).

Past	Positive	Negative
had to / didn't have to	obligation in the past I had to wear a school uniform when I was a child.	no obligation in the past We didn't have to go to school on Saturdays.
must*	changes to 'had to'	-
should have + pp / shouldn't have + pp	a past action which didn't happen: the advice / regret is too late You should have gone to bed earlier, now you have missed the train.	a past action which didn't happen: the advice / regret is too late You shouldn't have taken that job., it was a bad idea.

* Remember 'must have done' is a modal verb of deduction or speculation, not obligation in the past

Obligation

• We use have/has to for obligation.

we can also say I have got to … with the same meaning.

• Have to/must mean the same when we are talking about

rules and obligations, i.e. external obligation. But must

generally refers to internal obligation (i.e. the speaker feels

something is necessary).

• Must is not very common in question forms.

must is followed by the infinitive without to.

Advice

• Should/shouldn’t is used when we want to give a strong

opinion, telling someone the best thing to do. We can ask

for somebody’s opinion with Do you think I should … ? or

using the modal verb Should I … ?

• should/shouldn’t and had better are

followed by the infinitive without to.

• In the expression ought to, ought is always followed by to: He

ought to be here. Ought he to be here? This use of ought is very

formal, however, and should is more common in questions.

• Had better is a two-word modal verb and, like ought to,

does not have different tenses. We can contract ´had` (e.g.

you’d better). Had better has the same meaning as should

and ought to but it also implies more strongly that there

will be a problem if you don’t follow this advice.

Permission

• The modal verb can is used to convey the idea of permission

(can also conveys the ideas of possibility and ability). We can

also use could, may and might for permission.

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