A complex sentence joins an independent clause with a subordinate clause. We use a subordinating conjunction to join the two clauses in a complex sentence.
Here are the most common subordinating conjunctions: as, after, because, before, if, since, unless, until, when, while
Anytime you use one of these words to begin a clause, you are writing a subordinate clause. A subordinate clause cannot stand on its own; it must be joined with an independent clause.
Look at these subordinate clauses. Each begins with a subordinating conjunction. These clauses make no sense by themselves. Without an independent clause, these subordinate clauses are fragments.
As I was leaving the house
Before we go to bed
If I can help you
Unless we put some gas into the car
When the clock strikes twelve
Look what happens when we add an independent clause to each of these subordinate clauses. Now we have real sentences and not fragments.
As I was leaving the house, the smoke alarm started squealing.
Before we go to bed, we need to do some laundry.
Let me know if I can help you.
Unless we put some gas into the car, we won’t be able to drive to Wisconsin.
The carriage turns into a pumpkin when the clock strikes twelve.
A complex sentence combines an independent clause and a subordinate clause. A complex sentence will always have a subordinating conjunction in it.
▶It’s your turn. Add an independent clause to this subordinate clause to make a complex sentence. While we eat dinner Did you make a complete sentence? Good for you!