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I know the slight difference between should and ought to.

should is used when we want to express our subjective opinion.
ought to is used when we want to express an objective truth.


Then why there are sentences like:

I think we ought to do all we can.

And lots of other sentences starting with the phrase "I think we ought to"

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marjoonjanI know the slight difference between should and ought to.
should is used when we want to express our subjective opinion.
ought to is used when we want to express an objective truth.

This difference is not always recognised. I'm not sure to what extent it really exists. It may be more of a textbook rule than something that people follow in the real world.

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marjoonjanshould is used when we want to express our subjective opinion.
ought to is used when we want to express an objective truth.

I have never heard this. 'should' is used 25 times for every 'ought to' according to the Google Ngrams Viewer. I guess that means that, with regard to these two phrasings, we express an opinion 25 times more often than we express a truth. I wonder.

In any case, according to what I've read on the topic, 'should' and 'ought to' are equivalent in every way except that 'ought to' cannot be used with "evaluative" force, as 'should' can be. Thus:

It's surprising that he should say that. OK
It's surprising that he ought to say that. NO

CJ