Here's why:
* Subordinating conjunctions connect a dependent clause (which cannot stand alone as a complete sentence) to an independent clause (which can stand alone). They introduce a subordinate clause, showing its relationship to the main clause.
* Example: "Before you leave, make sure to lock the door."
* "Before you leave" is the dependent clause.
* "Make sure to lock the door" is the independent clause.
"Before" establishes a temporal relationship, indicating that the action in the dependent clause happens prior to the action in the independent clause.