A complex sentence joins a subordinate clause and an independent clause. A subordinate clause always begins with a subordinating conjunction. When you being a sentence with a subordinating conjunction, you must put a comma after the subordinate clause.
Here are some more subordinating conjunctions: since, unless, while, until, although. Anytime you begin a sentence with one of these words, you are writing a subordinate clause. That subordinate clause begins a complex sentence – and you need to put a comma after the subordinate clause.
Here are some complex sentences that begin with a subordinate clause followed by a comma:
Since we live in Florida, we are used to hot weather.
Unless you like to walk, we should probably take the car.
While you are at the mall, you should stop at the pet store.
Until the sun comes up, it will be too dark to drive.
Although the sun is shining, the air is still cool.
When you begin a sentence with a subordinating conjunction, you must add a comma and an independent clause to make a complete complex sentence.
It’s your turn. Choose one of these words to begin a sentence: since, unless, while, until, although. Remember to put a comma after the subordinate clause and then add an independent clause to complete the sentence.