A complex sentence has two parts: a subordinate clause and an independent clause.
The subordinate clause always begins with a subordinating conjunction.
Let me show you what that means:
The word when is a subordinating conjunction. Anytime you begin a sentence with when, you are writing a complex sentence.
Here is an example of a subordinate clause beginning with the word when:
When I finish my homework
This subordinate clause is not a complete sentence; it cannot stand alone.
Let’s add an independent clause to make it a complete sentence.
When I finish my homework, I can play video games.
When I finish my homework, I’ll eat a snack.
Now that we have added the independent clause, the sentence makes sense.
Notice that we put a comma between the subordinate clause and the independent clause.
Here are some more complex sentences with when:
When it gets dark, we can light a fire in the firepit.
When Dad gets home, we can pack the car for the trip.
When the timer rings, dinner will be ready.
The word if is another subordinating conjunction. Here are complex sentences beginning with if:
If it rains, our game will be postponed.
If I get off work early, we can go to the party.
If you get hungry, you can eat the leftover sub sandwiches.
The words when and if are subordinating conjunctions. They introduce a subordinate clause. You must add an independent clause to the subordinate clause to make a complete sentence. Use a comma between the two clauses.