A complex sentence is made by joining a simple sentence and a subordinate clause. When you begin a complex sentence with the subordinate clause, you must put a comma after the subordinate clause.
While you are making dinner, I will pay the bills.
The word while is a subordinating conjunction. The clause While you are making dinner is a subordinate clause. Because the sentence begins with the subordinate clause, we must put a comma after the clause.
If you begin with the simple sentence and then follow with a subordinate clause, do not use a comma.
I will pay the bills while you are making dinner.
The two sentences say the same thing. We have only changed the order of the two clauses.
Here are more examples. Notice that we only use a comma if we begin with the subordinate clause.
If the snow melts, we can take a walk. We can take a walk if the snow melts.
Because I lost my key, I can’t get into the house. I can’t get into the house because I lost my key.
After we clean the house, we can go to the mall. We can go to the mall after we clean the house.
Use a comma in a complex sentence when you begin the sentence with a subordinating conjunction. The comma must go after the subordinate clause. Do not use a comma in a complex sentence if the subordinating conjunction is in the middle of the sentence.
▶Now you try it. Insert a comma in this sentence if it is necessary. I am not hungry for dinner because I ate a snack this afternoon. Did you find the subordinating conjunction in the middle of the sentence? I hope you did not put a comma into this complex sentence.