完成体是动词的一种形式，表示动作或状况已经完成，结构是：主动词have的不同时态+-ed分词。可以分为目前完成体和过去完成体。目前完成体大伙比较熟知了，那样今天我们就和大伙一块学习大家不太熟知的过去完成体~
过去完成体，顾名思义，就是表示在过去某一时间之前已经发生或完成的动作，也就是大家所说的“过去的过去”。因此，它有“已完成”使用方法和“没有完成”使用方法。结构为：had+-ed分词。
1、“已完成”：表示一个动作或状况在过去某时之前已经完成或结束。
I had finished my homework when my friends came.我朋友来的时候，我就已经完成家庭作业了。
My father had left for Beijing before I came home. 在我到家前，父亲已经离开去北京了。
2、“没有完成”：表示一个动作或状况在过去某时之前已经开始，一直延续到这一过去时间，而且到那时还没有结束，仍有继续下去的可能性。
Jack said that he had made much progress in English since he came here.
杰克说自从他到这儿将来，英语已经获得了非常大进步。
By six o’clock these workers had worked twelve hours. 到六点钟时，这类工人已经工作十二时辰了。
3、连词引导过去完成体使用方法：有的连词引导的过去完成时可互换。
When I reached the station, the train had already left.
=I reached the station after the train had left.
=I didn’t reach the station until the train had left.
=The train had left before I reached the station.
在我到车站时，火车已经走了。
4、过去完成体的虚拟使用方法：表示与过去局势相反的主观设想。
1. 用于if, if only, as if引导的分句：
If you had worked hard, you would have passed the exam. 假如你好好学习的话，你就已经通过考试了。
If I were you, I would not do it. 我如果你，就不做这事。
If only I had finished it before. 如果我之前完成了这件事就好了。
I felt as if I had been a bird in the sky.我感觉自己宛若天空一只小鸟。
2. 用于I wish, I would rather引导的that从句：
I wished that I had knew the answer to the question before he asked me. 我期望在他问我以前，我了解问题的答案。
I would rather you had told me the truth. 我宁可你跟我说实话。
3. 用于expect, think, intend, mean, want, suppose等表示过去未曾达成的期望、计划或意图。
I had intended to buy a present for you, but I forgot that. 我本计划给你买礼物的，但我忘了。
I had hoped that we could pass the exam, but we failed in the end. 我本期望大家都能通过考试，但最后大家失败了。
因此，由上可知，比起目前完成体，过去完成体使用方法复杂，且具体“虚拟”的特殊使用方法，需要大家在平常用中加以注意。