結構：had+過去分詞
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引導的時間狀語從句放在句首要在句子后面加逗號，如果放在主句后則不用加。
1.變疑問句將助動詞移到句首
Had she finished her homework?
2.變否定句在助動詞后面加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.特殊疑問句：
What had she done?
