We can use have to + infinitive, must + infinitive and should + infinitive to express obligation (something you have to do).
|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:
- 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).
|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
|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
|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.
Would you like more practice? Get a new grammar lesson every week, a new listening lesson every week and more, Come to our academies!