Definition of ought to modal verb from the Oxford Advanced Learner's Dictionary

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ought to

modal verb
ˈɔːt tə
 
; ˈɔːt tə
 
before vowels and finallyˈɔːt tu
 
; ˈɔːt tu
 
negative ought not to short form (especially British English oughtn't to
 
1 used to say what is the right thing to doThey ought to apologize.‘Ought I to write to say thank you?’ ‘Yes, I think you ought (to).’They ought to have apologized (= but they didn't).Such things ought not to be allowed.He oughtn't to have been driving so fast.2 used to say what you expect or would like to happenChildren ought to be able to read by the age of 7.Nurses ought to earn more.3 used to say what you advise or recommendWe ought to be leaving now.This is delicious. You ought to try some.You ought to have come to the meeting. It was interesting.4 used to say what has probably happened or is probably trueIf he started out at nine, he ought to be here by now.That ought to be enough food for the four of us.Oughtn't the water to have boiled by now?
Usage noteUsage note: modal verbsThe modal verbs are can, could, may, might, must, ought to, shall, should, will and would. Dare, need, have to and used to also share some of the features of modal verbs.Modal verbs have only one form. They have no -ing or -ed forms and do not add -s to the 3rd person singular form:He can speak three languages. She will try and visit tomorrow.Modal verbs are followed by the infinitive of another verb without to. The exceptions are ought to, have to and used to:You must find a job. You ought to stop smoking. I used to smoke but I gave up two years ago.Questions are formed without do/does in the present, or did in the past:Can I invite Mary? Should I have invited Mary?Negative sentences are formed with not or the short form -n’t and do not use do/does or did.You will find more help with how to use modal verbs at the dictionary entries for each verb.Usage noteUsage note: should / ought / had betterShould and ought to are both used to say that something is the best thing or the right thing to do, but should is much more common:You should take the baby to the doctor’s. I ought to give up smoking. In questions, should is usually used instead of ought to:Should we call the doctor?Had better can also be used to say what is the best thing to do in a situation that is happening now:We’d better hurry or we’ll miss the train.You form the past by using should have or ought to have:She should have asked for some help. You ought to have been more careful.The forms should not or shouldn’t (and ought not to or oughtn’t to, which are rare in North American English and formal in British English) are used to say that something is a bad idea or the wrong thing to do:You shouldn’t drive so fast.The forms should not have or shouldn’t have and, much less frequently, ought not to have or oughtn’t to have are used to talk about the past:I’m sorry, I shouldn’t have lost my temper.