Glinda tells the Wicked Witch, "Be gone before somebody drops a house on you!" Which word in her sentence is a subordinating conjunction?

Before.

Explanation:

Subordinating conjunctions are those types of conjunctions that act as the joining words for an independent clause and a dependent clause. From the above provided sentence,

Glinda tells the Wicked Witch, "Be gone before somebody drops a house on you!"

the bolded words "Be gone" is the independent clause while the bold, italicized words "somebody drops a house on you!" is the dependent clause. And in between these two clauses is the word "before' which is a conjunction. This is the subordinating conjunction.