過去完成時的構成:had+動詞過去分詞
1.表示過去某一時刻或某一動作之前完成的動作或所處的狀態,常和「by/before+過去的時間」連用;
I had learned 1000 English words by the time I was ten.
By the time my parents reached home yesterday, I had cooked the dinner already.
2.表示在過去某一動作之前就已經發生或完成了的動作,常和「when/before」等引導的從句作為動作發生時間先後的對比,或通過上下文表示;
When I woke up,it had stopped raining.
Before she came to China,Marry had tought Chinese for about three years.