用法:在过去的时间里,两个动作中,发生在前的那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。