概念： 表示过去的过去，其构成是had +过去分词。
一、过去完成时常用的连词有： before, by, until, when, after, once, as soon as等。例如：
He said that he had learned some English before.
By the time he was twelve, Edison had began to make a living by himself.
Tom was disappointed that most of the guests had left when he arrived at the party.
1. 在told, said, knew, heard, thought等动词引导的宾语从句中，一般用过去完成时。例如：
She said (that) she had never been to Paris.
When the police arrived, the thieves had run away.
3. 表示意向的动词，如hope, wish, expect, think, intend, mean, suppose等，用过去完成时表示“原本……，未能……”。例如：
We had hoped that you would come, but you didn't.
The students ________ busily when Miss Brown went to get a book she ____ in the office.
A. had written, left
B. were writing, has left
C. had written, had left
D. were writing, had left
答案D。 “把书忘在办公室”发生在“去取书”这一过去的动作之前，因此“忘了书”这一动作发生在过去的过去，用过去完成时。句中when代表时间的一点，表示在“同学们正忙于……”这一背景下， when所引导的动作正在发生。因此前一句应用过去进行时。