By the time we arrived, the guests __________.

This sentence describes two past actions, where one action was completed before the other. "By the time we arrived" sets a specific point in the past. The action of the guests leaving happened before this arrival. The past perfect tense ("had left") is correctly used to show an action that was completed before another past action. "Left" (simple past) would imply they left at the same time or after we arrived. "Leave" (simple present) and "leaves" (simple present for third person singular) are incorrect as the events are in the past.