Here's why:
* Subordinating conjunctions connect a dependent clause to an independent clause.
* Dependent clauses cannot stand alone as a sentence.
* Independent clauses can stand alone as a sentence.
Example:
* Whenever I see you, I smile.
In this example:
* "Whenever I see you" is the dependent clause (it cannot stand alone).
* "I smile" is the independent clause (it can stand alone).
* "Whenever" connects the two clauses and makes the dependent clause depend on the independent clause.
Let me know if you'd like more examples!