We use of course to mean something like as everybody knows or as is obvious.
- It looks as if the sun goes round the earth, but of course the earth really goes around the sun.
Of course is not a polite reply to a statement of fact.
- ‘Could you help me?’ ‘Of course.’
- ‘It is cold.’ ‘It certainly is.’ (‘Of course it is’ would be quite rude, because it would suggest that the first speaker said something too obvious to be worth mentioning.)
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