虚拟语气的句型分类和用法
虚拟语气可以粗略地分为两大类：
第一类：should 型虚拟式 第二类：were 型虚拟式
1.should 型虚拟式：should + 原形动词(往往省去should, 特别是在考试时)
这类虚拟式最多用在以下词或句型中：
(1) It is necessary that…的主语从句中，如：
It is necessary that the chief should be present personally at the meeting.(主任有必要亲自参加这次会议。)
只要看到"necessary"，马上要想到这是"should型"虚拟式。
记住下面构成"should型"虚拟式的词：
essential (必要的) / important(重要的) / necessary(必要的) / imperative(迫切的)/ impossible(不可能的)/ natural(自然的)/ preferable(更好的)/ advisable(合理的)/ desirable(理想的)/ urgent(紧迫的)/ incredible(难以想象的)
(2) order (命令) / recommend(推荐)/ suggest(建议)/ demand(要求) / command(命令)/ propose(建议)/ require(要求)/ request(请求) / insist(坚持认为)/ urge(主张) / advise(劝告)/ ask(要求)/ prefer(宁愿)/ desire(愿望)/ move(提议)/ vote(提议)
由这些词构成的宾语从句，主语从句、表语从句、同位语从句或其他有关句型时，都要使用"should型"虚拟式，例如医学.全在.线.提供：
It was ordered that the medicines (should) be sent there by plane at once. (主语从句)
The order was that the medicines (should) be sent there by plane at once.(表语从句)
The chief ordered that the medicines (should) be sent there by plane at once.(宾语从句)
The order that the medicines (should) be sent there by plane at once was sent to the branch office.(同位语从句)
(3) 在 in case / lest = for fear that(以免… / 以防…)结构中，例如：
He took a raincoat with him in case it should rain (或用真实语气：…in case it rained)
She put a wool blanket over the child for fear that (lest) he should catch a cold. (她把一条毛毯盖在孩子身上，以防他感冒。)
可见，此类虚拟语气比较容易接受，考试时只要认准了上面的那些字一般就不难作出准确选择。
就解题而言，考生要注意：1)A的位置往往安排干扰项，如使用must, may, might, could等类词进行干扰;2)句中往往省去should, 直接用否定词not + 原形动词的形式，以"迷惑"考生，例如：
The housemaster(男舍监)was very strict.He asked that we ______ television on week nights.
A. must not watch
B. not watch
C. should not have watched
D. not be watching
该题中的"ask"意为"要求"，故是个"should型"虚拟式, 正确答案为B.
(4)在it is surprising, disappointing, a pity, a shame, no wonder, simply a miracle + 从句的结构中，表示惊讶、高兴或遗憾，例如：
It is surprising that she should look so miserable. (真是奇怪，她竟看上去如此不高兴。)
It is a great pity that you should miss such a good film. (你要是错过了这样好的电影那真是遗憾。)
