Sentence: "Please get to bed early since you have a test tomorrow."
Adverb Clause: since you have a test tomorrow.
Subordinating Conjunction: since.
Word(s) Modified: get to bed early.
In the given sentence, the adverb clause "since you have a test tomorrow" functions to provide a reason or cause for the action expressed in the main clause. The subordinating conjunction "since" introduces the adverb clause and shows the relationship between the main clause and the dependent clause. The word modified by the adverb clause is "get to bed early," as it indicates the time and manner in which the action should be performed.