The word that begins the subordinate clause is before. The word before, used in this sentence, is a subordinating conjunction, (ie a "joining word used to join a main clause to a dependent one) and its use indicates the start of a subordinate clause.
The reason for this is that "before" is only used at the start of a dependent or subordinate phrase. "Before she goes out" isn't a sentence, as the presence of the word "because" tells us that the sentence has another part, and the other part is where the main message ( main clause) is to be found. This is the real reason why we are told at school not to start sentences with "because" "and" or "but" - because these words are there to join two things together and therefore shouldn't be used to introduce the whole sentence. But I don't think that's so important!
The reason for this is that "before" is only used at the start of a dependent or subordinate phrase. "Before she goes out" isn't a sentence, as the presence of the word "because" tells us that the sentence has another part, and the other part is where the main message ( main clause) is to be found. This is the real reason why we are told at school not to start sentences with "because" "and" or "but" - because these words are there to join two things together and therefore shouldn't be used to introduce the whole sentence. But I don't think that's so important!