Here's why:
* Subordinate clause: It cannot stand alone as a complete sentence. It needs an independent clause to make sense.
* Dependent clause: It relies on an independent clause for its meaning.
* Type: It's an adverbial clause because it modifies the verb of the independent clause, telling us when the action of the independent clause takes place.
Example:
* When we finish dinner, we'll watch a movie. (The independent clause is "we'll watch a movie." The dependent clause tells us when this happens.)
Let me know if you'd like more examples or explanations!