A complex sentence is made up of an independent clause and a subordinate clause. It matters not whether you begin with the subordinate clause or the independent clause. The meaning of the sentence will remain the same.
Here is a complex sentence: If you open the door, I can carry this box inside.
Right now, it begins with the subordinate clause. If we change the order, the sentence will look like this:
I can carry this box inside if you open the door.
Even though we began the second sentence with the independent clause, the meaning of the sentence did not change.
Here are more examples of complex sentences. Notice that even though we change the beginning clause of each sentence, the meaning stays the same.
I can weed the garden while you wash the car.
While you wash the car, I can weed the garden.
Before it rains, I want to mow the lawn.
I want to mow the lawn before it rains.
In each of these examples, we switched the order of the independent and subordinate clauses, but we did not change the meaning of the sentence. However, we did change the punctuation. Notice that we only use a comma when we start the sentence with a subordinate clause.
The order of the clauses in a complex sentence can be changed without changing the meaning of the sentence.
Only use a comma between clauses when you begin the complex sentence with a subordinate clause.