I.基本概念：
过去完成时表示：在过去某个时间点以前或过去的某个动作之前已发生或完成的动作或状态。过去完成时主要用于以下的几种情况：
II. 基本用法：
1、用于动作之间的比较：如果一个动作或状态发生于一个过去的动作之前，须用过去完成时；
e.g. She had well prepared her lessons, so she got full marks in the exam.
I had fini shed my homework when I went to bed.
We had a lrea dy known the news before he told us about it.
2、直接引语是一般过去式和现在完成时，把它们转换成间接引语时，一般用过去完成时；
e.g. “I didn’t go home last Sunday,” she told me. →
She told me that she hadn’t gone home the Sunday before.
“We have already heard about the bad news,” they said. →
They said that they had already heard about the bad news.
3、当句子中含有 “by…” 引导的过去时间状语时；
e.g. I had grasped about 10000 English words by the end of 2000.
