if的虚拟条件句中，如果含有were，should，had等动词时，省略if并将were，should，had等词移到句首：这就是虚拟条件句的倒装。
虚拟条件句中含有had
例如：
If he had come yesterday，he would have know it.
=Had he come yesterday，he would have known it.
虚拟条件句中含有were
例如：
If I were there，I would help them.
=Were I there，I would help them.
虚拟条件句中含有should
例如：
If I should have 2000,000 yuan，I would buy a good car.
=Should I have 2000,000 yuan,I would buy a good car.
虚拟语气定义
虚拟语气主要用来表示假设，而非客观存在的事实，所陈述的是一个条件，不一定是事实，甚至完全与事实相反。虚拟语气通过谓语动词的特殊形式来表示。此外如需表达主观愿望或某种强烈的感情时，也可用虚拟语气。