A complex sentence combines a simple sentence and a subordinate clause. The first word of a subordinate clause is always a subordinating conjunction.
Here are some subordinating conjunctions: because, before, after, when, while
If you begin a sentence with one of these words, you have started with a subordinate clause. You must put a comma after the subordinate clause and then add a complete thought to the end of the sentence.
Subordinate clause: Before I eat dinner
Complete thought: I will take a shower.
Complex sentence: Before I eat dinner, I will take a shower.
Subordinate clause: While you are washing the dishes
Complete thought: I will take out the trash.
Complex sentence: While you are washing the dishes, I will take out the trash.
After you wash the car, we should water the flowers.
When it gets warmer, we can plant the garden.
Whenever you start with a subordinating conjunction, you are writing a subordinate clause. Always use a comma after a subordinate clause and then finish with a complete thought.
▶It’s your turn. This sentence begins with a subordinate clause. Where should you put a comma? After the sun goes down it will feel cooler. Did you figure it out? Good for you.