A subordinating conjunction is used to connect a subordinate clause (or dependent clause) to an independent clause. Some examples include: when, unless, what, if, whenever, whatever, who, whoever, that, because, and since. To help me remember subordinating conjunctions, I try to think of words that I would use to help answer a question.
Here is a sentence that includes an example of a subordinating conjunction:
I washed the dishes since my brother agreed to put them away.
The first part of the sentence (I washed the dishes) could stand by itself and is therefore an independent clause. It could also function as a simple sentence because it does contain a subject and a verb.
A dependent or subordinate clause must begin with a subordinating conjunction and include both a subject and a verb. It should feel (with the conjunction included) that it cannot stand alone. Since my brother agreed to put them away feels like it needs more to be finished or a complete thought.
Here are other examples of subordinating conjunctions at work.
Whatever we do over the weekend doesn't matter to me.
My doctor, who I completely trust, has chosen to give me a dose of medicine.
Hope that helps.