Here's why:
* Subordinating conjunctions connect a dependent clause to an independent clause. The dependent clause cannot stand alone as a complete sentence.
* "Until" introduces a dependent clause that tells us when something will happen or stop happening.
Example:
* I will wait until you arrive.
* "I will wait" is the independent clause (can stand alone).
* "until you arrive" is the dependent clause (cannot stand alone).
* "Until" connects the two clauses and shows the time relationship between them.
Let me know if you'd like more examples!