Ought was originally the past tense of owe; but now it generally points to Present and Future time. It differs from other Auxiliaries in that it is followed by the to-form of the Infinitive (e.g. ‘You ought to do it.’) and not the simple form (e.g. ‘You must do it.’)  Ought is not as forceful as must, but is stronger than should.

Ought expresses ideas such as duty, necessity, fitness, moral obligation, etc.

They ought to help him. (duty)
He ought to attend office regularly. (duty)
He ought to be ashamed of himself. (fitness)
You ought to obey your parents. (duty)
We ought to help the poor. (moral obligation)
You ought to visit Kashmir before leaving India. (fitness)

When ought refers to past time, it is followed by the Perfect Infinitive:

He ought to have helped his friend.
We ought not to have wasted so much time over it.
We have done things which we ought not to have done.
We have left undone things which we ought to have done.

Exercise

Complete the following sentences using an appropriate auxiliary verb.

1. ………………….. I open the window?

2. You …………………. not to waste your time.

3. She ………………….. to have sent that application form yesterday itself.

4. The doctor said that I …………………. quit smoking.

5. ………………. I wait any longer?

Answers

1. Should I open the window?

2. You ought not to waste your time.

3. She ought to have sent that application form yesterday itself.

4. The doctor said that I must quit smoking.

5. Should / need I wait any longer?