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is a group of words which has ‘one main clause’ and ‘one or more subordinate clauses’. The main clause is used without any conjunction but the subordinate clause must be used with any of the subordinate conjunctions, such as because, as, so, when, who, which, if, unless, where, so that, so…that, though, although, as if, as though, etc.
He went back home early because he was tired.
‘He went back home early’ — main clause
‘because’ — subordinate conjunction (included in the subordinate clause)
‘because he was tired’ — subordinate clause (reason)