英语语法分析,虚拟语气The millions of calculations involved,had they been done by hand,all practical value would have lost by the time they were finished.为什么这句话是虚拟语气,为什么had they been done by hand 要倒装

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英语语法分析,虚拟语气
The millions of calculations involved,had they been done by hand,all practical value would have lost by the time they were finished.
为什么这句话是虚拟语气,为什么had they been done by hand 要倒装

(If )they had been done by hand 省略了If作状语