过去完成时的结构及用法
过去完成时所涉及的动作需在过去某个特定时间之前发生，也就是发生于“过去的过去”。
过去完成时的构成
过去完成时是通过 “had+过去分词”的形式构建的。
例句：
Had he gone home when you arrived?
当你抵达时，他已经回家了吗？
By 5:00 yesterday morning we had done that work.
截止昨天早上 5 点，我们已经完成了那份工作。
The train (had) started before I reached the station.
在我抵达车站之前，列车就已经出发了。
She said she had made much progress since she came here.
她说自她来到这儿以后，已经取得了极大的进步。
