一、虚拟语气用于表语从句及同位语从句中
当advise, decision, desire, demand, idea, order, motion, necessity, insistence, instrution, plan, preference, proposal, pray, recommendation, requirement, resolution, suggestion, understanding等名词后接接表语从句或同位语从句时，从句的谓语用“should+动词原形”的形式来构成虚拟语气。
例：The professor gave order that the test be finished before 7:30.
教授指示考试须在七点半以前结束。
例：The suggestion that the mayor present the prizes was accepted by everyone.
二、虚拟语气用于方式状语从句中
在as if, as though 引导的方式状语从句中，虚拟语气的形式是：用过去时表示与现在事实相反或或对现在情况有所怀疑；用过去完成时表示对过去事实的假设。
例：It seems as if it were spring.
现在好像是春天似的。
例：She appeared as if she had known nothing about it.
她看上去对此一无所知。
三、虚拟语气用于目的状语从句中
在lest, for fear that, in case 等引导的目的状语从句中，谓语动词用“should+动词原形”来构成虚拟语气；在so that, in order that 等引导的目的状语从句中，谓语动词除了用“should+动词原形”外，还可以用“might, could+动词原形”来构成虚拟语气。
例：She’s studying hard for fear that he should fall behind.
她努力学习，生怕跟不上。
四、虚拟语气用于让步状语从句中
在让步状语从句中，谓语动词多用动词原形来构成虚拟语气。
注：由even if 或even though引导的让步状语从句中，其从句的谓语动词形式与含有非真实条件从句的主从句中的谓语动词一样来构成虚拟语气。

经典练习：
1、Our idea is that the match____.
A)would put off
B)ought to put off
C)be put off
D)could put off
2、One of the requirements for a fire is that the material_____to its burning temperature.
A)is beated
B)will be heated
C)be heated
D)would be heated
3、That tree looked as if it _____for a long time.
A)hasn’t watered
B)didn’t water
C)hadn’t been watered
D)wasn’t watered
4、The business of each day, _____selling goods or shipping them, went quite smoothly.
A)it being
B)be it
C)was it
D)it was

参考答案:

1、答案选C)。根据虚拟语气的语法要求,that 从句中的动词用动词原形或“should+动词原形”的形式。所以C为正确选项。
2、答案选C)。requirement 后接一表语从句，从句的谓语用“should+动词原形”的形式来构成虚拟语气，四个选项中只有C符合。句意：产生火的必要条件之一是物质被加热到它的燃点。
3、答案选C)。as if引导一个表示与过去事实相反的虚拟语气的句子。主句的谓语动词是一般过去时，所以as if 后虚拟语气的谓语动词要用过去完成时，所以C是正确的。
4、答案选B)。be it selling goods or shipping them 是表示让步意义的虚拟倒装句，它是由whether it (might)be selling goods or shipping them这个句子省略whether后再将be 移至主语之前得来的。其它几项均不符合句子结构。句意：无论是售货还是运货，每天的业务都很不错。