为什么这些句子需要虚拟语气?它们变成虚拟语气又有什么用?

虚拟语气用来表示说话人的主观愿望或假想,而不表示客观存在的事实,所说的是一个条件,不一定是事实,或与事实相反.虚拟语气通过谓语动词的特殊形式来表示.
1.在whatever,whichever,whenever,whoever,
wherever,however,no matter wh-word 等引导的让步状语从句中,从句虚拟语气结构为：
① may +动词原形(指现在或将来).如：
We will finish it on time no matter what / whatever
may happen.不管发生什么事,我们都要按时完成.
2.在though,although等引导的让步状语从句中,从句虚拟语气结构为 should +动词原形,主句结构不限.如：
Although / Though
he should often be late,he is a good student.尽管他经常迟到,他还是个好学生.
3.原因状语从句中的虚拟语气
amazed,angry,
annoyed,astonished,disappointed,frightened,happy,pleased,proud,sorry,
surprised,upset等后面的状语从句中常用虚拟语气.其虚拟语气的结构为：
① should + 原形动词(指现在或将来).如：
He was angry that
you should call him by name.
他很生气,你竟然对他直呼其名.
I was astonished
that he should not answer such an easy question.我很惊讶他竟答不出如此简单的问题.
4.
② should + 完成式,指过去.如：
I’m very sorry
that you should have failed the exam.
我很遗憾,你这次考试竟然失败了.
虚拟语气有很多