什么是过去完成时？
过去完成时是英语中用来描述在过去某个时间之前已经完成的动作、事件或状态的时态。它通常用来表达一个过去的时间点之前的动作。以下是一些例子，展示了过去完成时的用法：
- 过去的时间点之前已经完成的动作：
- By the time I arrived, they had already left.（我到达时，他们已经离开了。）
- She had finished her work before the meeting started.（会议开始前，她已经完成了工作。）
- 过去完成时与过去某个时间点的关系：
- He had visited that museum before, so he didn’t go again.（他以前去过那个博物馆，所以没有再去。）
- She had read the book, so she knew the story.（她已经读过那本书，所以知道故事情节。）
- 过去的先后发生：
- After he had eaten dinner, he went for a walk.（他吃完晚餐后，出去散步了。）
- They had finished the project before the deadline.（他们在截止日期之前完成了项目。）
- 过去的情况与另一个过去的时间点：
- She had lived in that city before she moved here.（在搬到这里之前，她住在那个城市。）
- They had studied English before they traveled to the United States.（在去美国旅行之前，他们学过英语。）
过去完成时用来描述一个过去时间点之前已经完成的动作，特别是在谈论两个过去事件的顺序、先后关系或发生情况时。
1. 过去完成时的形式:
had + 过去分词
2. 过去完成时的用法:
表示事情或动作发生在过去某时间之前。
* Most of the guests had left when he arrived at the party. 当他到达晚会时，他发现大多数客人已经离开了。
* I had learned 900 English words by the time I was ten. 我十岁之前已经学会了900个单词。
* How many houses had they built in their village by the end of last year? 到去年年底他们村里建了多少房子？
* We had reached the station before nine o’clock. 我们9点之前已经到达了车站。
* Until then, his family hadn’t heard from him for six month. 到那时为止，他们父母半年没有收到他的来信了。
* Tom flew home, but his father had already died.汤姆乘飞机回家，他的父亲却已经去世了。
3. 过去完成时的一些常见情况
1) 用在宾语从句中
* She said she had been a doctor for twenty years.她说她当过20年的医生。
* We were surprised at what she had done.我们对于她的所作所为感到惊讶。
* He asked me where I had been during the summer holidays. 他问我暑假去哪里了。
* Our teacher told us that Columbus discovered America in 1492. 我们老师告诉我们哥伦布于1492年发现了美洲。
* The news came as no surprise to me. I had known for some time that the factory was going to shut down. 听到这个消息我并不感到吃惊。工厂要倒闭这件事我早就知道了。
* She wondered who had left the door open.她想知道谁敞着门的。
2) 用在状语从句中
* When the police arrived, the thieves had run away. 警察到达时，小偷们早就跑了。
* They began to climb the mountain after they had bought all the food and drinks. 他们买了食物和饮料后就开始爬山了。
* He had worked in the factory for three years before he joined the Army. 他参军之前已经在工厂里干了3年。
* We had got to the station before the train left. 火车开出之前，我们就已经到了车站。
* Helen had left her keys in the office so she had to wait until her husband came home. 海伦把钥匙忘到办公室里了，因此她不得不等她丈夫回来。
* He described the scene as if he had been there.他描绘的景色如同他去过那里一样。
3) 用在定语从句中
* His eyes shone brightly when he finally received the magazine he had long expected. 当他终于收到他盼望已久的杂志时，他兴奋得两眼闪光。
* I returned the book that I had borrowed. 我已经归还了我借的书。
4) 用于 hope, intend, mean, think 等动词表示未实现的愿望。
* They had hoped to be able to arrive before ten. 他们本来打算能在10点之前到达。
* I had intended to come over to see you last night，but someone called and I couldn’t get away. 昨晚我本想去看你的，但有人来了我走不开。
* We had meant to tell her the news but found that she wasn’t in.我们本想把这个消息告诉她的，但发现她不在家。
* I thought that I had seen him before. 我觉得我以前见过他。
