When a complex sentence begins with a subordinate clause, we put a comma after the clause.
If it rains tomorrow, we will cancel the picnic.
However, if the complex sentence begins with an independent clause followed by the subordinate clause, no comma is needed.
We will cancel the picnic if it rains tomorrow.
Look at these sentence examples. The first sentence begins with a subordinate clause and uses a comma after the clause. The second sentence begins with an independent clause, and no comma is needed.
When you are hungry, you can eat. You can eat when you are hungry.
Because we are late, we missed the bus. We missed the bus because we are late.
Before it gets dark, you should mow the lawn. You should mow the lawn before it gets dark.
When a complex sentence begins with a subordinate clause, we must put a comma after the subordinate clause. No comma is needed if the subordinate clause is at the end of the sentence.
▶Now you try it. Decide which sentence needs a comma after the subordinate clause. After I swim I’ll take a nap. I’ll take a nap after I swim. Did you put a comma into the first sentence? Good for you!