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I ought to help him .

I ought to have helped him.

Shall I use (have + past participle) for the past form of ought to?

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"Ought to" is a modal verb. Modal verbs have only one form. They have no -ing or -ed forms. 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.

Questions are formed without do/does in the present, or did in the past. Negative sentences are formed with not or the short form -n’t and do not use do/does or did.

In case of "ought to" form the past by using "ought to have". For example: You ought to have been more careful.

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    But "I ought to have helped him" is not a full counterpart of "I ought to help him", because the present-tense form allows for the possibility that I will help him, while the ought to have form indicades that I did not. If we say "I was under an obligation to help him", this will leave open the possibility that I did in fact help him. – CowperKettle Dec 19 '15 at 8:45
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    I ought to help him is the right thing to do, we are clear about that, but I ought to have helped him shows that I didn't do the right thing but did the wrong. It works for the past but not in the right way. – Alejandro Dec 20 '15 at 18:20
  • "ought to" already implies that you aren't currently helping, and can only begin to in the (possibly very near) future. – chepner Oct 4 '16 at 17:55
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"Ought to" is a semi-modal, I would say. Modal verbs (can, shall, may) have very clear past tense forms (could, should, might). "Ought to" is not nearly as clear.

I would expect "ought to" to be considered a past tense form that, like "must," has no present tense form. My reasoning is that "ought to" has the same meaning as "should," which is a past tense form. Experts (who clearly know more than I do, as linguistics is pretty far out of my area!) tend to take an opposing view and consider "ought to" to be a present-tense form. http://www.tandfonline.com/doi/abs/10.1080/00138388408598359?journalCode=nest20

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