It's vital that he speak to the registrar as soon as possible. 此...答: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...