It is necessary that everything be ready by three o'clock.在台湾的高中，很多英
文老师会把这句话解释成"should"被省略。30年前，台北有位高中补教名师郑蓉就告诉学
生，他问了well-educated American，结果对方说，不是省略should，是根本就没有
should。这种用法是present subjunctive!
It is necessary that everything be ready by three o'clock.(Ammerican usage)
It is necessary that everything should be ready by three o'clock.(British
usage)