It is required that the machine be oiled as frequently as necessary
问题描述：
It is required that the machine be oiled as frequently as necessary
为什么用Be oiled而不用Must be oiled ,is oiled或者Will oil求详解
答
本句为虚拟语气, 在这里require , suggest 用法是类似的,都是虚拟语气required 加从句要求从句that 主语+ （should）+谓语 所以这句话实际上是machine should be oiled as frequently as necessary句中恰恰省略...