Dolly must clean up her room before she goes out. in this sentence, what word begins the subordinate clause

I believe it is: before she goes out

The main clause is "Dolly must clean up her room" which is what the sentence is primarily about. "Before she goes out" depends on the main clause, making it the dependent clause. "Before" is the first word in the dependent clause and it is a preposition making it the word that begins the subordinate clause. 

Not sure if there was any sort of picture or something to go along with this but my guess is the book went into deep depth and was very informative.