Modal Verbs of ObligationWe 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)
|must / mustn’t||strong obligation (possibly based on the speaker’s opinion)
|should / shouldn’t||mild obligation or advice
||mild negative obligation or advice
Mustn't means it's not allowed, or it's a bad idea:
- You mustn't eat so much chocolate, you'll be sick
- 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
||no obligation in the past
|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
||a past action which didn’t happen: the advice / regret is too late
* 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.
Should, Ought to BY WWW.ECENGLISH.COMFor giving advice or expressing a conclusion 'should' and 'ought to' are interchangeable. They are used to express the same ideas.
You should/ought to stop smoking. He has been working on the project all week. He should/ought to be ready by this evening.
Should is also used in hypothetical situations: Should anyone call, take a message.
Call me should you need any help.
Should is used with 'imagine', 'say' and 'think' to give a tentative opinion: I should think it will take us an hour to drive to Rome.
Need'Need' expresses necessity: You need to exercise more often. You're putting on weight.
You don't need to wear a tie if you don't want to.
Needn't have/Didn't need to'Needn't have' is used to talk about an action that was done but was unnecessary. We needn't have rushed to the airport. The flight was delayed.
I needn't have brought an umbrella. It didn't rain.
‘Didn't need to' is used to talk about an action that wasn’t done because it was not necessary. I didn't need to call Mike. I met him in the street.