RG9u10
By the time I got outside, the bus had already left.
Grammar
过去完成时定义：过去完成时表示在过去某一时间或动作之前已经发生或完成了的动作，也可表示在过去某一时过去完成时间开始到过去另一时间仍未结束。
过去完成时结构：had +pp (动词的过去分词）后常跟由by引导的过去时间状语或before when 引导的含有一般过去时的时间状语从句。在时间上含有”过去的过去“之意。
1.By the time she went outside, the bus had already left.
2.When we got there,she had left her backpack at home.
Function
Narrate past events.
---What happened ?
---I overslept. And by the time I got up, my brother had already gotten into the shower.
资料看点
1.The bus had already left.(一般问句，附加问句，否定句，肯否回答）
2.I left my book at home . I realized.(合并句子）
3.They got married three years ago.(用for/since改写)