scarcely had she fallen asleep when a knock at the door awakened her...为什么用had这个过去完成时

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翻译：她几乎已经睡着了，当敲门声唤醒了她...这是考查when引导的时间状语从句，主句用了wakened一般过去时，所以从句用过去完成时had done形式，表示她睡着的动作是发生在敲门之前
表示过去的过去，所以用had done形式

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有一个固定用法是scarcely +完成时（还必须是倒装句）when...
此处when 引导的状语从句是一般过去时，所以scarcely前面的主句用过去完成时

正“hardly(scarcely)…when(before)…”(或“no sooner…than…”)结构在英语中用得十分频繁.通常,此结构中主句的谓语动词用过去完成时,从句用一般过去时.正如Practical English Usage 中所说:“Hardly(and scarcel...

scarely.....when..., hardly.....when.... No sooner....than....
以上三个表达法都表示“一......就.....”常用在过去完成时，并用在倒装句中。例如：
他一下火车就去看那个受伤的士兵。
Scarely had he got off the bus when he went to visit the wounded soldier.
Hardly had he got off the bus when he went to visit the wounded soldier.
No sooner had he got off the bus than he went to visit the wounded soldier.