A complex sentence links two or more ideas within one sentence. The two parts, or clauses, are usually linked by a conjunction. In the following sentence, the conjunction because is used to link the two clauses.
Mr Jones forgot to collect his suitcase because he was in a hurry.
The main clause in a complex sentence makes sense on its own.
Mr Jones forgot to collect his suitcase because he was in a hurry.
The subordinate clause begins with the conjunction and does not make sense on its own.
Mr Jones forgot to collect his suitcase because he was in a hurry.
A subordinate clause can be at the beginning of a sentence as well as at the end. Moving the clauses around can make your writing more varied and interesting to read.
Because he was in a hurry, Mr Jones forgot to collect his suitcase.
If the subordinate clause is at the beginning of the sentence we usually use a comma to separate the clauses.