What Is A Subordinating Conjunction?
A subordinating conjunction is a word that connects an independent clause to a dependent clause. An independent clause can stand alone as a sentence. In other words, it does not need any additional information to operate as a sentence. The sentence "The student failed the test" is an example of an independent clause.
A dependent clause adds extra information to the main clause. These clauses cannot stand by themselves and their meaning is dependent on the independent clause. They are not complete sentences. For example, "because she didn't study" is not a complete sentence.
However, combine the two clauses, and we have "The student failed the test because she didn't study." A complete idea has been expressed and enough information has been presented to fully explain the thought. What joined the two clauses? The word "because." And there we have our first subordinating conjunction.