概念：表示在过去某一时间或动作之前已经发生或完成了的动作，即过去的过去（ past-in-the-past ）”。
构成：助动词 had + 过去分词”，其中 had 通用于各种人称。
They had already had breakfast before they arrived at the hotel.在到达旅馆之前，他们已经吃过了早饭。
She had finished writing the composition by 10：00 this morning. 今天早晨10点之前，她已经写完了作文。
