It'svitalthathespeaktotheregistrarassoonaspossible.此句中为什么是speak不是speaks?

Question

It's vital that he speak to the registrar as soon as possible.此句中为什么是speak不是speaks?

Answer 1

因为 前面句子中有 vital 后面要用虚拟语气,即：
It's vital that 主语 + （should） do should 可以省略
类似用法还有：
在It is (was)important (necessary,desirable,imperative,advisable,incredible,urgent,possible,essential,natural,preferable,insistent,crucial,better,best,ridiculous,vital) that…
及It is (was) a pity (shame,wonder,must,suggestion,proposal,requirement,request,desire,order,recommendation) that….,It is (was)desired (arranged,decided,ordered,proposed,requested,recommended,suggested,settled) that…等句型的主语从句中,其谓语用“should+动词原形(其中should可以省略)”来构成虚拟语气.
例：It is important that we (should)speak politely.
我们说话要有礼貌,这是很重要的.