在英语中，大致分为三种语气，即陈述语气、祈使语气和虚拟语气。陈述语气用来陈述事实；祈使语气用来生发命令以及请求等；虚拟语气表示说话人所说的话不是事实，而是一种假设、愿望、怀疑或推测。
首先，让我们来学习规则的虚拟条件句，即主从句虚拟的时间一致。请看下表：
记下这张表，就为我们学习虚拟语气奠定了基础。怎样才能快速地记下它呢？我们先从主句的谓语动词形式开始，虚拟将来和现在的形式都是would/should/might/could do,虚拟过去的形式是would/should/might/could have done.我们再来看从句的谓语的动词形式,其实质就是在陈述语气下所使用的谓语动词形式相应的过去,即一般变过去､过去变过去完成｡当我们虚拟的时间是将来时,在陈述语气中,我们用do/shall do或者am/is/are to do来表示将来,在虚拟语气中,我们就将其变为相应的过去,即为do→did,shall do→should do,am/is/are to do→were to do;如果虚拟的时间是现在,在陈述语气中我们常用do来表示现在,在虚拟语气中即为do→did;如果虚拟的时间是过去,在陈述语气中,我们常用did来表过去.在虚拟语气中即为did→had done.请看下面的例子:
(1)如果他有钱,他就会买车｡(虚拟现在)
在陈述语气中,我们可译为:
If he is rich,he will buy a car.
在虚拟语气中,我们就将is→were,即为:If he were rich,he would buy a car.
(2)如果我有时间,我就要学法语｡(虚拟现在)
If I had time,I would study French.(have→had)
(3)如果再来一次洪水,他就会意识到他的错误｡(虚拟将来)
If there should be another flood,he might realize his mistake.(shall→should)
(4)如果今天晚上你错过那场电影,你就会感到遗憾｡(虚拟将来)
If you missed the film tonight,you would feel sorry for it.(miss→missed)
(5)如果我早一点离开,我就会赶上那列车｡(虚拟过去)
If I had left a little earlier,I would have caught the train.(left→had left)
(6)如果我当初听你的建议,我就不会犯错误了｡(虚拟过去)
If I had taken your advice,I wouldn,t have made a mistake.(took→had taken)
最后,我们来学习一下混合虚拟条件句,即主句和从句虚拟的时间不一致,我们可以按各自不同的时间去套表中对应的谓语动词形式｡请看下面的例子:
①如果他多得到6票,他现在就是我们的主席｡
从句虚拟过去,主句虚拟现在,我们可译为:
If he had received six more votes,he would be our chairman now.
②如果玛丽今天到的话,那她必定三天之前就出发了｡
从句虚拟将来,主句虚拟过去,我们可译为:
If Mary should arrive today,she must have started 3 days ago.