过去完成时
用法：在过去的时间里，两个动作中，发生在前的哪个动作要用过去完成时。
结构：had+过去分词
After she had finished her homework, she went shopping.
They had sold the car before I asked the price.
The train had left before I arrived at the station.
After/before引导的时间状语从句放在句首要在句子后面加逗号，如果放在主句后则不用加。
★ 变疑问句将助动词移到句首
Had she finished her homework?
★ 变否定句在助动词后面加not
She hadn't finished her homework.
★ 肯定回答及否定回答
Yes, she had. / No, she hadn't.
★ 特殊疑问句
What had she done?