过去分词。had done用于过去完成时，过去完成时即“过去的过去”，过去已经发生过的事。例句：By the end of last term，we had learned 5000 new words.到上学期末，我们已经学了5000个新词。
had后面用动词过去分词形式，即had done。
had done是过去完成时。表示动作发生在过去的过去。通常句中有过去时间状语作参照。
例句：I had finished my homework before my father came back.
在我父亲回来之前，我已经完成了家庭作业。
had done句型的例句They had never seen so many of the locusts before.
（他们以前从未见过如此多的蝗虫）
I had reached the station before six o'clock.
（六点之前我已到达车站。）
Fortunately, the population had left the village before the volcano erupted.
（幸运的是，火山喷发前人们已离开了村庄。）
