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。
1.变疑问句将助动词移到句首
Had she finished her homework?
2.变否定句在助动词后面加not
She hadn’t finished her homework.
3.肯定回答及否定回答
Yes, she had. No, she hadn’t.
4.特殊疑问句：
What had she done?
