Here's why:
* Subordinating conjunctions connect a dependent clause (one that can't stand alone) to an independent clause (one that can).
* The dependent clause introduced by "if" expresses a condition.
* The independent clause provides the result or consequence of that condition.
Example:
* If it rains, we'll stay inside.
Breakdown:
* If it rains is a dependent clause (it can't stand alone).
* We'll stay inside is an independent clause (it can stand alone).
* "If" connects the two clauses and shows that staying inside depends on whether it rains.