A complex sentence has two parts: a subordinate clause and an independent clause.
The subordinate clause always begins with a subordinating conjunction.
Let’s look at what that means.
The word since is a subordinating conjunction. When you begin with the word since, you are writing a subordinate clause. Since today is Tuesday
That clause makes no sense by itself. The subordinate clause needs an independent clause attached to it.
Look what happens when we add an independent clause:
Since today is Tuesday, we have a student government meeting after school.
Since today is Tuesday, we’ll put the trash out for tomorrow’s pickup.
Anytime a sentence begins with since, we attach an independent clause so the sentence makes sense.
That’s how we write a complex sentence.
Since I don’t speak Spanish, I couldn’t understand what the man said.
Since we don’t have practice today, I’ll go to the mall.
The word because is another subordinating conjunction. Anytime you begin a sentence with because, you are writing a complex sentence.
Because I forgot my lunch at home
This subordinate clause cannot stand alone as a sentence. We add a comma and an independent clause so that the sentence makes sense.
Because I forgot my lunch at home, I had to go hungry.
Because I forgot my lunch at home, I had to use bus money to buy food.
Here are more sentences beginning with the subordinating conjunction because:
Because I broke my arm, I couldn’t complete the volleyball tryout.
Because I have such a busy fall schedule, I didn’t sign up for driver’s ed.
Anytime you begin a sentence with since or because, you are starting a subordinate clause. To write a complete sentence, you must put a comma after the subordinate clause and add an independent clause. That’s how we write a complex sentence.