虚拟语气是动词的一种特殊形式，用来表达说话人的愿望、请求、意图、建议、惊奇、设想等。虚拟语气在各种结构中的运用是历年三级统考必考的内容，考生必须熟练掌握。以下从五个方面介绍。
(一)虚拟条件句中虚拟语气的运用：
虚拟条件句从时间上又分为与现在事实相反，与过去事实相反，与将来事实可能相反三种情况。表示与现在事实相反的，if从句用过去式，主句用would (或could ,should ,might)加原形动词。与过去事实相反的，if从句用过去完成时，主句用would(或could ,should ,might)+have done结构。与将来事实可能相反的，if 从句用should (或were to )加动词原形，主句用would加动词原形。
1.If a better material _____, the strength of the part would have been increased.
A. had been used
B. had been using
C. being used
D. using
(根据前面的讲解，正确答案为A。) (1992年58题)
2.Had the weather been good, the children _____ out for a walk.
A. had gone
B. could have gone
C. would go
D. went
(答案：B。与过去的事实相反。当if从句中含有were, had, should这三个词时，if可以省略，主谓倒装。)(2001年57题)
(二)某些动词后的宾语从句以及某些名词后的表语或同位语从句中虚拟语气的运用，这些动词或名词包括：
suggest (suggestion),propose(proposal),advise (advice)demand, insist, order, request, require, recommend, desire, ask, decide等。在这些从句中，谓语形式为should加动词原形，should可以省略。
例：The generals command was that the soldiers _____ their fort and carry out more important task.
A. would leave
B. leave
C. left
D. have left
(答案：B) (2002年49题)
(三)在It is desired (或desirable), It is important等结构后面的主语从句中，动词用原形或should+原形动词。这些结构有：
It is suggested, It is requested,It was ordered, It is necessary, It is essential, It is vital, It is urgent, It is impossible, It is preferable, It is advisable, It was proposed等。
例：Its desired that she _____ to teach us at least twice a week.
A. comes
B. will com
C. come
D. may come
(答案：C)(1997年29题)
(四)would rather, as if/though 以及wish后边that从句中虚拟语气的运用。
1.I am too busy these days. I would rather all of you _____ next month for a dinner.
A. come
B. would come
C. came
D. have come
(答案：C。would rather 后面的从句中，动词用过去式。)(2002年46题)
2.He talks as if he _____ everything in the world.
A. knows
B. knew
C. had known
D. would have know
(正确答案为B。在as if/though后边的方式状语从句中，表示与现在事实相反或对现在的情况有所怀疑，动词用过去式;如果表示的是想象中的过去的动作，用过去完成时。本句表示对现在的情况有所怀疑。)(1998年45题)
3.You look as if you had seen a ghost.
(此句中as if 后边是说话人想象中的动作，所以用过去完成时。)
4.I wish I knew his address.
(在wish后边的从句中，如果指现在或将来的愿望，动词用过去式。这句话告诉我们说话人并不知道他的地址。)
5.Peter wishes that he _____ law instead of literature when he was in college。
A. could study
B. studied
C. had studied
D. would study
(答案为C。表示过去没有实现的愿望，动词用过去完成时。)(2000年32题)
(五)在It is (high) time后边的that从句中，动词用过去式，表示该做某事了。
例：Dont you think it is time you _____ smoking?
A. give up
B. gave up
C. would give up
D. should give up
(答案为B。)(1999年31题)