過去完成時 
 用法:在過去的時間里,兩個動作中,發(fā)生在前的哪個動作要用過去完成時。
 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。
 ★ 變疑問句將助動詞移到句首
     Had she finished her homework?
 ★ 變否定句在助動詞后面加not
   She hadn't finished her homework.
 ★ 肯定回答及否定回答
   Yes, she had. / No, she hadn't.
 ★ 特殊疑問句
   What had she done?