shall should may might can could must虚拟语气的用法与区别比较

情态动词所表达的可能性程度：must/can't  should/shouldn't  might/may (not)
另外两个"类情态词的形式："need/needn't; have to/don't have to
 最自然的虚拟状态：由should/would+原型时态（不含时间只含状态）
本质上是过去将来时：即,时间固定在过去将来,状态不同：一般、进行、完成、完成进行.
这时"虚拟语气"的产生往往是因为我们要表达"本来应该……"（而现在却还没有……）
（本来可以……,本来能……）
I should go! (… but I'm still here!) (一般)
I should be working now! (进行)
I should have practiced more (than I did)! (完成)
我应该多多练习!（言下之意,现在我练习得不多.）
I shouldn't dream away my time too much! (完成的否定)
(actually I did dream away my time too much!)
It shouldn't have been leaking for such a long time! (完成进行)
I may/might/could have finished! (完成)
一些常见的句型中,就会出现这种虚拟语气,而处于从句之中,should 常常被省略掉
o suggest, advise, propose, recommend, plan;
o demand, order, direct, arrange, command, decide;
o require, request;
o think, expect, believe, insist, suspect.
由于他们的含义中包含"建议,假设,应该"这类的含义,所以,由他们引起的从句中,就会包含有should+原型时态构成的虚拟语气.
这些动词（以及他们的名次形式,分词形式）引起的从句还有其他的变形：
主语从句,表语从句,同位语从句
It's suggested that…
My suggestion is that…
The only suggestion that...
The only suggestion I can give you now is that…
一些形容词引起的表语从句中,也会有同样的情况
important; necessary; essential
It's natural ; strange; incredible that
a pity; a shame; no wonder
 由lest, for fear that, in case 引起的从句中多使用should
 表达与事实相反
1. 与现在相反：使用[过去时]：
I wish I were not here! (一般现在一般过去)
Suppose we were not here.
He loved me as if I were his own son. (一般现在一般过去)
Hope I weren't always losing things! (现在进行过去进行)
If only/If I hadn't been there! (现在完成过去完成)
What if I hadn't been waiting right here! (现在完成进行过去完成进行)
常考句型：It's (high) time (that)…; would rather (that)…
这两个从句,只能表达对现在的看法,所以,从句中只有一般过去时.
2. 与过去相反：过去完成时；
How nice it is if I had past the test!
How nice it is if I had slept a little more this morning!
3. 与将来相反?将来的事情没有发生,所以只能推测.
If it rains tomorrow, we'll have to stay one day more.
不过,由于可以用be to表示将来；所以,虚拟语气中经常出现were to；也是CET-4的常考语法点.
 虚拟条件句
o if 部分,做一个与事实相反的假设（所以只有一般过去和过去完成）；
o 主句部分,这是表示基于这个假设的推测,一般使用情态动词would,少数情况下使用could/might/may.
o 注意：两个部分之间,是有逻辑关系,而在两部分的谓语动词时态上,没有必然的联系.
 注意,虚拟条件句中的if可以省略,造成were/had提前,产生倒装.
 隐含的非真实条件
What would you do with 50 thousand dollar?
How could I be happy without you?
除了条件状语从句之外,原因状语从句也会出现虚拟语气.
o 由in order that, so that引起的从句,肯定的时候可以使用may/might; can/could; 否定的时候,多用shouldn't;
o whoever, whatever, no matter what引起的从句中,多用may+
情态动词的基本用法及其区别
最近几年高考试题中常常借助语境来考查情态动词的基本用法及其区别,因此在平时学习时准确理解和掌握情态动词的基本用法十分重要.情态动词的用法复杂多变,在高考试题中,命题者常常利用语境和句子之间意义上的细微差别来考查学生对情态动词的理解和掌握.对于情态动词,除了要求考生能够准确掌握它们的基本用法外,还要充分利用高考试题所设置的语境来分析句子之间所体现的特殊关系.下面就近几年来高考试题中出现的情态动词的考点进行归纳分析,以便同学们复习掌握.
一、用“情态动词＋have ＋done”结构表示对过去动作的推测,高考试题中常用过去时态或过去的时间状语给以暗示.情态动词的这一用法可以用 “对立统一”来概括.
1．当试题的前句和后句在动作和意义上相互补充说明,且整个句意在动作和时间上是一个整体时,我们可用“统一”关系来解决这样的试题.常见的结构有：
must have done：
表示对过去动作的肯定推测,常译作“一定做了……”,只能用于肯定句中.其否定形式为can’t／couldn’t have done
疑问式为Can／Could...have done?.
could ／might have done：表示对过去发生的动作的可能性推测,常译作“可能做了……”.如：
1) My sister met him at the Grand Theater yesterday afternoon, so he
＿＿＿＿＿ your lecture． 
A． couldn’t have attended
B． needn’t have attended
C． mustn’t have attended
D． shouldn’t have attended
本题选A.
2) Jack ＿＿＿＿ yet, otherwise he would have telephoned me．
A． mustn’t have arrived
B． shouldn’t have arrived
C． can’t have arrived
D． need not have arrived （C）
2．当试题的前后句在动作和意义上构成转折关系时,常借助“but, however, instead”等词来表示过去的动作与客观事实不符,这时我们就可以用“对立”关系来解决这样的试题.这种结构常见的有：
should have done ／ ought to have done：表示过去本应该做某事而实际上没有做.
should not have done ／ ought not to have done：表示过去本不应该做某事但事实上却做了.
need have done：表示过去本来有必要去做某事,但事实上没有做.
need not have done：表示过去本来没有必要做某事,但事实上却做了.如：
3) I was really anxious about you． You ＿＿＿＿＿ home without a word．
(NMET2001)
A． mustn’t leave B． shouldn’t have left
C． couldn’t have left D． needn’t leave
“本不应该离家出走却走了”,故本题选B.
4) I told Sally how to get here, but perhaps I ＿＿＿＿＿ for her．
(NMET’94)
A． had to write it out
B． must have written it out
C． should have written it out
D． ought to write it out
由句中的连词but可知前后句之间是对立关系,分析题意可知本题应选C.
二、考查情态动词基本用法之间的比较和辨析.最近几年高考试题中常借助具体的语境来考查考生对那些最常见的情态动词的基本用法的理解和掌握,因此在做这样的试题时应认真分析语境中所含的实际意义,并结合情态动词的基本含义和用法做出正确的选择.
5) —Is John coming by train?
—He should, but he ＿＿＿＿＿＿ not． He likes driving his car．
A． must B． can C． need D． may
mustn’t 表示“禁止、不准”；cannot 表示“不可能”；need not 表示“不必要”；may not
表示“可能不”.分析语境可知本题应选D.
6) —I hear you’ve got a set of valuable Australian coins． ＿＿＿＿＿＿ I have a look?
—Yes, certainly．
A． Do B． May C． Shall D． Should
分析语境可知这是在征求对方的许可,may表示“允许、可以”,语气比较委婉 shall常用于第一、三人称作主语的疑问句中,表示征求对方意见和指示,如果此空用shall,则意为“要（我）看一下吗?”,不符合上下文意思.故本题选B.
7) Mr Bush is on time for everything． How ＿＿＿＿ it be that he was late for the opening ceremony?
A． can B． should C． may D． must
must be 表示肯定的猜测,只能用于肯定句中,由题意可知本题应选A.
8) —Are you coming to Jeff’s party?
—I’m not sure． I ＿＿＿＿ go to the concert instead．
A． must B． would C． should D． might
由题意和下句中的 “I’m not sure”
可知这段对话中存在一种可能性推测,might可以用来表示一种比较委婉的可能性判断,故本题选D.又如： I should have been there, but I ＿＿＿＿＿ not find the time．
A． would B． could C． might D． should
分析题意可知第二个分句表示过去的某种能力；C 项只表示语气上的可能性,与题意不符.故本题选B.
9) Johnny, you ＿＿＿＿ play with the knife, you ＿＿＿＿ hurt yourself．
A． won’t; can’t B． mustn’t; may
C． shouldn’t; must D． can’t; shouldn’t
mustn’t 表示“不可以；禁止”,分析题意可知第二个空表示某种可能性,故本题选B.
10) —Will you stay for lunch?
—Sorry, ＿＿＿＿＿＿． My brother is coming to see me．
A． I mustn’t B． I can’t
C． I needn’t D． I won’t
分析题意可知因为“我弟弟要来看我”,所以“不能留下”,因此对别人的邀请或要求应给予礼貌的拒绝.A 项表示“禁止”；C项表示“不必要”；而D项表示“不会”,均不符合题意.故本题选B.又如：
—Could I borrow your dictionary?
—Yes, of course you ＿＿＿＿＿．
A． might B． will C． can D． should (C)
11)—When can I come for the photos? I need them tomorrow afternoon．
—They ＿＿＿＿＿ be ready by 1200．
A． can B． should C． might D． need
该题考查情态动词should的基本含义,分析句意可知本题应选B.又如：
12) The fire spread through the hotel very quickly but everyone ＿＿＿＿ get out．(
A． had to B． would C． could D． was able to
该题考查了could和be able to的区别,二者都可表示过去时间的能力,但如果表示过去成功地做了某事只能使用was ／ were able to do,故本题选D.
13) —Shall I tell John about it ?
—No, you ＿＿＿＿＿． I’ve told him already．
A． needn’t B． wouldn’t C． mustn’t D． shouldn’t
情态动词shall在试题中表示征询对方意见或请求指示.答句暗示 “没有必要了”,故本题选A