1. 虚拟条件句中的倒装结构 虚拟条件句中如果含有were，had或should，可以把它们提前到句首，省去连词if，变成倒装句。如： （1）与过去事实相反：Had he not been promoted (＝If he had not been promoted), he would never have remained with the company.（如果他的职位没有得到晋升，他早就不在这个公司了。） （2）与现在事实相反：Were he indoors（=If he were indoors）, he would’t feel so cold.（他要是在室内就不会觉得冷了。 （3）对将来的推测：Should you change your mind（=If you should change you mind）, no one would blame you.（如果你要改变主意，没有人会怪你。）
2. 含蓄虚拟语气 有些句子中没有明显的虚拟条件句，而是利用其他手段来暗示虚拟条件的存在。这类句子大都表示与过去事实相反的情况。常用来表示这种含蓄虚拟语气的词有： but for, without, under … condition，or, but, otherwise, or, unfortunately等，如：Without（=If there had not been）your help, I wouldn’t have achieved so much.（要不是有你的帮助，我不会取得这么大的成就。）Under more favorable conditions（=If we had had more favorable conditions）, we could have done better.（如果条件再好点，我们会做得更好。）I would have rung you up but the telephone was out of order（=if my telephone had not been out of order）（我本可以给你打电话的，可是没想到电话坏了。）
3.“should / shouldn’t / ought to / ought not to /need/needn’t+ have+过去分词”用于表示与过去事实相反的虚拟语气，意为“应该做而未做 / 不应该做却做了”。如He oughtn’t to have been complaining about such a trivial matter.（他本不应该为这件小事而抱怨不休。）
