When a complex sentence begins with a subordinate clause, the clause must be followed by a comma.
When the rain stops, we will have recess.
For variety, we can also begin a complex sentence with the independent clause, followed by the subordinate clause. In this case, we do not put a comma in the sentence.
We will have recess when the rains stops.
Here are more examples. Notice the subordinate clause in bold lettering. Only use a comma after the subordinate clause.
If you have a dollar, I can give you a soda.
I can give you a soda if you have a dollar.
While I make dinner, you can set the table.
You can set the table while I make dinner.
In a complex sentence, only use a comma if the sentence begins with a subordinate clause.
It’s your turn. Does this complex sentence need a comma? Before you leave the house be sure to put away your clothes. Did you put a comma in here? Good for you!