A complex sentence combines a simple sentence with a subordinate clause. The simple sentence can stand on its own as a complete thought. The subordinate clause begins with a subordinating conjunction, and we must use a comma after the subordinate clause.
Here is a simple sentence: The baseball game is canceled.
Here is a subordinate clause: Because it is raining.
The word because is a subordinating conjunction.
When we use a subordinating conjunction, we are writing a subordinate clause. When we begin a sentence with a subordinate clause, we are writing a complex sentence. We must place a comma after the subordinate clause.
Because it is raining, the baseball game is canceled.
A subordinate clause begins with a subordinating conjunction. Anytime we begin a sentence with a subordinate clause, we must use a comma after the subordinate clause.
▶It’s your turn. This sentence begins with a subordinate clause. Decide where the comma should go. Because the tire is flat I cannot ride my bike. Did you put a comma after flat? Good for you!