过去完成时：

1.概念：以过去某个时间为标准，在此以前发生的动作或行为，或在过去某动作之前完成的行为，即“过去的过去”。

2.时间状语：before, by the end of last year(term, month…),etc.

3.基本结构：had + done.

4.否定形式：had + not + done.

5.一般疑问句：had放于句首。

6.例句：As soon as we got to the station, the train had left.

By the end of last month. We had reviewed four books