See a brief explanation of the basic structure of all modal auxiliary verbs...
Should and have to have similar meanings and are often interchangeable:
You ought to / should do what you can.
You should /ought to go and see that film. It's really good.
Marïa should have done it / ought to have done it.
Should / Ought to + infinitive is used to give strong advice or talk about obligation in the present or future. When these modal auxiliaries are used in the third person (he, she, they), then they can also sound like opinions of the speaker:
She should / ought to leave straight away.
We should / ought to do it now.
They shouldn't / oughtn't to leave tomorrow; they haven't finished yet.
Both should and ought to are used to talk about strong probability in the present or future. These probabilities are events we hope will happen, we do not use this structure for events we prefer not to happen
If you take 20 euros, that should / ought to be enough money.
I can hear a car. That should / ought to be Mary coming home.
It should / ought to be sunny tomorrow so I think we can go to the beach.
Compare with when we mention a probability we do not like:
It'll probably rain tomorrow so I don't think we can go to the beach (not, it should rain / it ought to rain...)
The structure should / ought to + have + past participle expresses a past obligation that was not done and we feel that it was wrong that it was not done:
He should have / ought to have finished it before (but he didn't do it).
I should have / ought to have paid the bill (but I didn't pay it).
There is a subtle difference between should and ought to. We use should when are expressing our opinion about what somebody's obligation or we are giving advice. We use ought to when we talk about duty, the rules or moral obligation:
You should study more if you want to pass your exams. (advice, opinion)
I think he should go on a diet. He's too fat. (opinion)
We ought to go and visit that poor old lady. She's all alone. (moral obligation)
You ought to recycle bottles, cans and paper. It's our duty. (duty, moral obligation)
You shouldn't smoke in here. It's not allowed. (rules)
It is possible to use should with the expression in case. We can also use should by itself to mean in case if we use inversion between should and the subject:
I looked out of the window in case she should be coming to see me (or, ...should she be coming to see me).
Let's buy some more beer in case we should need it (or, ...should we need it).
If we study the following sentences, we can see how English avoids the subjunctive by using should:
They were tied well so that they shouldn't fall.
He insisted that the men should be taken away.
Was it necessary that my friends should be told?
I'm surprised that she should shout at you.
Is it normal that an adult should be so jumpy?
The use of should as auxiliary in the above sentences sounds more elegant than simply using the verb: Is it normal that an adult is so jumpy?; I'm surprised that she shouted at you. The other possibility is the use of the infinitive (actually, the English subjunctive), which sounds archaic: Was it necessary that my friends be told?* By using the modal auxiliary should, we have an acceptable English both for written and spoken situations.
*However, the subjunctive used in this way is heard in US English.
Exercises on should and ought to...
Explanation of modal auxiliary verbs in general...
Modal auxiliary verbs 1 (can, could, may, might)...
Modal auxiliary verbs 3 (must)...
Modal auxiliary verbs 4 (would)...
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