A complex sentence is a sentence that contains one main (independent) clause and at least one subordinate (dependent) clause.
The subordinate clause depends on the main clause to complete its meaning.
These clauses are usually joined by conjunctions like because, although, when, which, or that.
Example: I stayed at home because it was raining.
Here, “I stayed at home” is the main clause and “because it was raining” is the subordinate clause explaining the reason.
Complex sentences help to express detailed and clear ideas.