完成体是动词的一种形式，表示动作或状态已经完成，结构是：主动词have的不同时态+-ed分词。可以分为现在完成体和过去完成体。现在完成体大家比较熟悉了，那么今天就和大家一起学习我们不太熟悉的过去完成体~
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，也就是我们所说的"过去的过去"。因此，它有"已完成"用法和"未完成"用法。结构为：had+-ed分词。
一、"已完成"：表示一个动作或状态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，爸爸已经离开去北京了。
二、"未完成"：表示一个动作或状态在过去某时之前已经开始，一直延续到这一过去时间，而且到那时还未结束，仍有继续下去的可能性。
Jack said that he had made much progress in English since he came here.
By six o’clock these workers had worked twelve hours.
三、连词引导过去完成体用法：有些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.
四、过去完成体的虚拟用法：表示与过去事态相反的主观设想。
1. 用于if, if only, as if引导的分句：
If you had worked hard, you would have passed the exam.
If I were you, I would not do it. 我要是你，就不做这事。
If only I had finished it before. 要是我之前完成了这件事就好了。
I felt as if I had been a bird in the sky.我感觉自己好似天空一只小鸟。
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.
I would rather you had told me the truth. 我宁愿你跟我说实话。
3. 用于expect, think, intend, mean, want, suppose等表示过去未曾实现的希望、打算或意图。
I had intended to buy a present for you, but I forgot that.
I had hoped that we could pass the exam, but we failed in the end.
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