Here's why:
* Subordinating conjunctions connect a dependent clause (a clause that cannot stand alone as a sentence) to an independent clause (a clause that can stand alone).
* Before introduces a dependent clause that expresses time or sequence.
Example:
* Before *we left the house, we packed our bags.*
In this sentence:
* "Before we left the house" is a dependent clause (it cannot stand alone).
* "We packed our bags" is an independent clause (it can stand alone).
* "Before" connects these two clauses and shows the order of events (packing bags happened before leaving the house).