Should and ought to are both modal verbs and are used to give advice or to express expectations. Although they can be used with the same meanings, should is used more in everyday English, whereas ought to has a more formal usage.
1 giving advice
You can say you should or you ought to when you are giving someone advice.
I think you should go see your doctor.
I think you ought to try a different approach.
2 expectation
You use should or ought to to say that you expect something to happen.
We should be there by dinner time.
It ought to get easier with practice.
You use should or ought to with ‘have’ and a past participle to say that you expect something to have happened already.
Be careful to use ‘have’ and not ‘of’. Don’t say, for example, ‘You should of heard by now that I’m OK’.
You should have heard by now that I’m OK.
It’s ten o’clock, so they ought to have reached the station.
You also use should or ought to with ‘have’ and a past participle to say that something was expected to happen, but did not happen.
Bags which should have gone to Rome were sent to New York.
The project ought to have finished by now.
Be Careful! You must use ‘have’ and a past participle in sentences like these. Don’t say, for example, `The project ought to finish by now’.
3 moral rightness
You can also use should or ought to to say that something is morally right.
Crimes should be punished.
I ought to call the police.
4 negative forms
Should and ought to have the negative forms should not and ought not to.
This should not be allowed to continue.
They ought not to have said anything.
The ‘not’ is not usually pronounced in full. When you write down what someone says, you write shouldn’t or oughtn’t to.
You shouldn’t speak like that, Andrew.
They oughtn’t to mention it.
When you make a negative statement with ought in American English, you can omit to:
You oughtn’t answer the door without your shirt on.
