It was a dangerous excursion, did he but know it, for its headquarters was the focus of the ‘military resistance’ which would strike against him in July 1944.
请教一下各位老师，看到海外平台说这个结构的意思等于"if he only knew it".
那样did he but know it 是不是是if省略的结构？可是if不是需要有had,should,were时才能省略并引起倒装吗？这种使用方法是不是合理？或者是不是是一种古老的使用方法？
期望老师解惑，谢谢！
虚拟条件句省略 if 产生的倒装，容易见到的有三种，就是网友说的这三种状况。
其他功能词放在主语之前也是可以的，事实上可以有七种形式： did, were, should, had, could, might, would + 主语 + 其他，几乎所有些 if虚拟句都可以如此倒装。比如：
Did we know her circumstances, we could do something to help her. 倘若目前大家了解她的处境，大家本来可以采取手段帮助她的。（此句为与目前状况不符的虚拟语气）
