Answer:
The passive voice is a type of sentence structure that emphasizes the object of an action instead of the subject performing the action. In the passive voice, the object of the sentence receives the action and the subject is omitted. For example: "The ball was thrown by him."
In the sentence "I tried to change the order, but they had already sent the books.", the passive voice is used to emphasize that the books were already sent rather than focusing on who sent them. The passive voice phrase in this sentence is "they had already sent the books."