Modal Verbs of Obligation

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

Modal Verbs of Obligation
Modal Verbs of Obligation
PresentPositiveNegative
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’tstrong obligation (possibly based on the speaker’s opinion) I must study today.negative obligation
You mustn’t smoke here.
should / shouldn’tmild 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:

  • You mustn’t eat so much chocolate, you’ll be sick

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).

PastPositiveNegative
had to / didn’t have toobligation 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. For example: Julie must have left. Her coat’s not here. See modals of probabilty for more information.

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