Question

He is not a thief. He is an imposter (join with coordinating conjunction)
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Answer 1

He is not a thief but an imposter.

A conjuction is used to connect 2 word, phrases or sentences.

Answer 2

coordinating conjunction are 'and', 'but' and 'or'.

It joins the words that link together parts of a sentence.

It is used to join together two clauses in a sentence.

→He is not a thief but an imposter.

Here 'But' is used to connect two sentences.