Should Ought

Should and ought have more or less same meaning in present day English and can be used interchangeably. Should, however, is more common than ought. Both should and ought are used to suggest obligation.

Examples:

He should spend more time on his homework.

He ought to spend more time on his homework.

You should not smoke so much.

You ought not to smoke so much.

Note that ought is always followed by the full infinitive with the sign to expressed.

Should and ought have a slightly negative force, suggesting that, although one has the obligation to do a certain thing, one may not do it. Should and ought thus differ from must, which is definitely positive in meaning and has almost the force of a command.

Both should and ought to have a past tense form and are frequently used in the past. This past tense form is obtained by the use of the auxiliary verb have and the past participle of the main verb.

Note that should and ought have definitely negative force in these past tense forms, suggesting that someone should have done something that was not done or should not have done something that was done.

Examples:

You should have sent them a telegram (but you did not).

I should have studied before my examination (but I did not).

You ought not to have said that to him.

John went to the movie last night but he should have stayed at home and prepared his lessons.