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。
Jack said that he had made much progress in English since he came here.
By six o’clock these workers had worked twelve hours. 到六点钟时，这些工人已经工作十二小时了。
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.
1. 用于if, if only, as if引导的分句：
If you had worked hard, you would have passed the exam. 如果你努力学习的话，你就已经通过考试了。
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. 我希望在他问我以前，我知道问题的答案。
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. 我本打算给你买礼物的，但我忘了。
I had hoped that we could pass the exam, but we failed in the end. 我本希望我们都能通过考试，但最终我们失败了。