虚拟语气中含蓄条件句的本质是什么?

提到条件句,人们一般会想到if引导的条件句,而英语中某些假设的条件句不是通过if从句表达出来,而是包含在某些短语、上下文或其他方式中,其谓语也常用虚拟语气,我们称此种结构为含蓄条件句.
（人教版）高中教材Book Ⅵ. Unit Ι. Art Page 3,课文最后一段出现了这样一个句子：“But without the impressionists, many of these painting styles would not exist.”由句意可以看出,此处应是一个条件句,而且暗含虚拟语气,这里假设的条件没有通过条件从句表达出来,而是隐含在without介词短语中,故名含蓄条件句.译作：但是如果没有印象派,那么这许多不同的风格就不可能存在.
又例如：Without water, there would be no life. 如果没有水,就不会有生命.
但可作含蓄条件句的并非without一种,我们就试着找出以下几种可作虚拟语气的含蓄条件句,以飨读者.
Ⅰ. But for the meeting, I would have gone home at the time.
要不是开会,我早就回家了. (but for 暗示了条件)
Ⅱ. It would be only partly right to follow in this way.
如果用这种方式,仅仅对了一半. （动词不定式暗示了条件）
Ⅲ. Having known in time ,we might have prevented the accident.
要是及时得知的话,我们也许能阻止这场事故.
（-ing 分词做条件状语,暗示条件：If we had kown in time）
Ⅳ. Given more attention, the tree could have grown better.
如果多留心的话,这树本来可以长的更好.（MET 1990）
(-ed 分词做条件状语,暗示: If the tree had been given more attention)
Ⅴ.I was too busy at that time, otherwise, I would have called you.
我当时太忙,否则我就给你打电话了.
（副词otherwise暗示条件）
由此可知,在特定的上下文或一目了然的情况下,if条件句可以省略,或使用介词短语、副词或非谓语动词等形式来代替if条件句,这种虚拟语气表达形式叫做含蓄条件句
所以,含蓄条件句的本质就是隐含if的虚拟语气句