结构：主语+助动词(had)+动词的过去分词。
用法
- 表示在过去某一时间或动作以前完成了的动作，即“过去的过去”，常用by, before等介词短语或一个时间状语从句来表示。例如：
- I had finished my homework by 6 o’clock yesterday. 我昨天六点钟之前就完成了作业。
- He had left before I arrived. 我到达之前他就离开了。
- 表示在过去某一时刻之前发生的动作或状态持续到过去某个时间，即“从过去到过去”，常与for+时间段，since+时间点或从句结构连用。例如：
- She had lived in Beijing for five years before she moved to Shanghai. 她搬到上海之前在北京住了五年。
- He had been ill since last week until he recovered yesterday. 他从上周开始就一直生病，直到昨天才康复。
- 表示由于过去的某个动作或状态而造成的结果或影响。例如：
- He was very hungry because he hadn’t eaten anything all day. 他很饿，因为他一整天什么都没吃。
- She was very happy because she had passed the exam. 她很高兴，因为她通过了考试。
- 表示在过去某个动作之前已经发生或完成的动作，常用在told, said, knew, heard, thought等动词后的宾语从句（或间接引语）中。例如：
- He told me he had seen the film. 他告诉我他看过这部电影。
- She thought he had gone to bed. 她以为他已经睡觉了。