A complex sentence is made up of a subordinate clause connected to an independent clause.
Sometimes the subordinate clause comes first in the sentence. Then the sentence begins with a subordinating conjunction. Here are some examples of a complex sentence with the subordinate clause at the beginning:
If it rains, we will cancel the picnic.
While you were sleeping, I baked a cake.
Before you go to school, we should eat breakfast.
When a complex sentence begins with a subordinate clause, we must put a comma after the clause.
Other times, the independent clause comes first in a complex sentence. Look at these examples of a complex sentence where the independent clause comes first:
We will cancel the picnic if it rains.
I baked a cake while you were sleeping.
We should eat breakfast before you go to school.
You can see that the dependent clause still starts with a subordinating conjunction, but when the independent clause comes first in a complex sentence, there is no comma.
A complex sentence uses a subordinating conjunction to join an independent clause and a subordinate clause. Use a comma after the subordinate clause if the subordinate clause comes first in the sentence.
▶It’s your turn. Make these two clauses into a complex sentence. Only use a comma if you begin with the subordinate clause. We can take a walk. After the rain stops. Did you make a good sentence? That’s great!