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We can use have to + infinitive, must + infinitive and should + infinitive to express obligation (something you have to do).
Present
Have to / don't have to (sometimes 'have got to')
Positive: strong obligation (possibly from outside)
- Children have to go to school.
Negative: no obligation
- I don't have to work on Sundays.
- You don't have to eat anything you don't like.
Must / mustn't
Positive: strong obligation (possibly based on the speaker's opinion)
- I must study today.
Negative: negative obligation
- You mustn't smoke here.
Positive: mild obligation or advice
- You should save some money.
Negative: 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).
Past
Had to / didn't have to
Positive: Obligation in the past
- I had to wear a school uniform when I was a child.
Negative: No obligation in the past
- We didn't have to go to school on Saturdays.
Positive: Obligation in the past
- I had to wear a school uniform when I was a child.
Negative: No obligation in the past
- We didn't have to go to school on Saturdays.
Positive: A past action which didn't happen so the advice / regret is too late
- You should have gone to bed earlier, now you have missed the train.
- 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.
- Julie must have left. Her coat's not here.
See modals of probabilty for more information.
- Click here to download this explanation as a pdf.
- Click here for modal verbs of obligation exercise 1.
- Click here for all the modal verbs exercises.