Ought to | Modal auxiliary verb

Ought is a modal auxiliary verb. There is no -s in the third person singular.

She ought to understand. (NOT She oughts to ...)

Ought is different from other auxiliary verbs. It is used with to

We ought to respect our parents.
We ought to help the poor.

Note that to is dropped in question tags.

You ought to love your country, ought not you? (NOT...ought not you to.)

Ought does not have infinitives (to ought) or participles (oughting, oughted). Questions and negatives are made without do.

Ought we to help them? (NOT Do we ought to ...)
You ought not to go now.

Ought is rarely used in questions and negatives; should is generally used instead. A structure with think ... ought is also common.

We ought to help them, shouldn't we? (More natural than ought not we?)
Do you think I ought to consult a doctor? (More natural than Ought I to consult a doctor?)
Should we tell her? (Less formal than Ought we to tell her?)

Ought: Meaning

Ought expresses duty, necessity, desirability and similar ideas. It is often used to advise people - to tell them that they have a duty to do things. The meaning is similar to should

You ought to attend office regularly. (Duty)
We ought to help the needy. (Moral obligation)
We ought to buy some furniture. (Necessity)

Ought is not as forceful as must.

Uses

To express probability

Ought can express logical probability.

If he started an hour ago, he ought to be here soon.

Ought to have + past participle

When ought refers to past time, it is followed by the perfect infinitive. This structure can be used to talk about things which were supposed to happen but did not.

I ought to have written to my parents, but I forgot.
You ought to have invited her to your party.

This structure can also be used to make guesses.

It is ten o'clock. He ought to have reached home.

Ought not to have can be used to talk about things that happened unnecessarily.

We ought not to have wasted so much time over it.
We have done things that we ought not to have done.
We have left undone things that we ought to have done.

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