Here's why:
* Subordinating conjunctions introduce dependent clauses (clauses that cannot stand alone as a complete sentence).
* Dependent clauses rely on independent clauses (clauses that can stand alone) to make a complete thought.
Example:
* If you study hard, you will succeed.
* If is the subordinating conjunction introducing the dependent clause "if you study hard".
* The independent clause is "you will succeed."
Together they form a complete sentence.