Ought to is a semi-modal verb because it is in some ways like a modal verb and in some ways like a main verb. For example, unlike modal verbs, it is followed by to, but like modal verbs, it does not change form for person:
I ought to phone my parents.
It ought to be easy now.
Ought to: form
Ought to comes first in the verb phrase (after the subject and before another verb):
We ought to do more exercise.
Ought to cannot be used with another modal verb:
Medicine ought to be free.
Medicine ought to can be free. or Medicine can ought to be free.
The negative is formed by adding ‘not’ after ought (ought not to). It can be contracted to oughtn’t to. We don’t use don’t, doesn’t, didn’t with ought to:
We ought not to have ordered so much food.
We don’t ought to have ordered so much food.
You oughtn’t to have said that about his mother.
You didn’t ought to have said that about his mother.
The negative of ought to is not common. We usually use shouldn’t or should not instead:
You shouldn’t speak to your father like that. (preferred to You oughtn’t to speak …)
The subject and ought to change position to form questions. We don’t use do, does, did:
Ought she to call the police?
Does she ought to call the police?
Ought we to be more worried about the environment?
Do we ought to be more worried about the environment?
The question form of ought to is not very common. It is very formal. We usually use should instead.
Ought to: uses
What is desired or ideal
We use ought to when talking about things which are desired or ideal:
They ought to have more parks in the city centre.
We ought to eat lots of fruit and vegetables every day.
We use ought to have + -ed form to talk about things that were desired or ideal in the past but which didn’t happen. It can express regret:
We ought to have locked the gate. Then the dog wouldn’t have got out. (The ideal or desired thing was that we locked the gate, but we didn’t.)
I often think that I ought to have studied medicine not pharmacy. (I would be happier now if I had studied medicine.)
What is likely
We can use ought to when we talk about what is likely or probable:
The concert ought to only take about two hours so we’ll be home by 12 pm.
There ought to be some good films at the cinema this weekend.
Ought to or should?
Ought to and should are similar in meaning. Should is more common than ought to. Ought to is more formal than should:
There ought to be more street lights here. (means the same as There should be more street lights here.)
I really ought to walk my dog more. He’s so fat. (means the same as I really should walk my dog more. He’s so fat.)
In speaking, we normally use should as a tag for clauses with ought to:
There ought to be a speed limit here, shouldn’t there? (preferred to There ought to be a speed limit here, oughtn’t there?)
We ought not to have to pay for basic medicines, should we? (preferred to We ought not to have to pay for basic medicines, ought we?)