A complex sentence joins an independent clause and a subordinate clause. The subordinate clause begins with a subordinate clause word (subordinating conjunction).
Some Subordinating Conjunctions: after, although, because, before, if, since, unless, until, when, where, while, who
A subordinate clause cannot stand alone. Here are two subordinate clauses; each begins with a subordinate clause word.
If it rains today
Because I lost my keys
These clauses are not complete sentences. You must put an independent clause after each subordinate clause to make a good sentence.
When you start with the subordinate clause, put a comma after the subordinate clause.
If it rains today, my ballgame will be cancelled.
Because I lost my keys, I’m locked out of the house.
You can also put the independent clause first. When you start a complex sentence with an independent clause, do not use a comma to separate the clauses. Look at these examples:
My ballgame will be cancelled if it rains today.
I’m locked out of the house because I lost my keys.
These sentences give the same information, but we switched the order of the clauses, and there are no commas.
Only use a comma when you begin a complex sentence with a subordinate clause.