Past perfect tense
had + 动词的过去分词 (p.p.)
定义：
过去完成时：
表示在过去的某一时间或动作之前已经发生或完成了的动作。即：过去的过去。
1.在老师进入教室前, 李磊擦了黑 板.
LiLei ______________ (clean) the blackboard by the time the teacher came into the classroom.
had cleaned
2. 当我们到那儿时火车已经离开了。
The train __________ (leave) when we arrived there.
had left
构成：
“过去的过去（ past-in-the-past ）”。　　　　　
past in the past
past
now