Here's why:
* Subordinating Conjunctions connect a dependent clause (which cannot stand alone as a sentence) to an independent clause (which can). They show the relationship between the two clauses.
* "When" introduces a dependent clause that expresses time.
For example:
* When the sun sets, the sky turns orange. ("When the sun sets" is the dependent clause, and "the sky turns orange" is the independent clause.)
Here are some other examples of how "when" is used as a subordinating conjunction:
* When you are finished, let me know.
* I will call you when I get there.
* The party was over when the music stopped.