在现在时态句里,用情态动词(Modal Verb)的过去时态
(could,might,should,would)表示说话人的谦虚、客气、有礼、委婉等：基本可互换,但might很少用
1. Would you be kind enough to show me the way to the post office.
(情态动词 would,表示客气有礼)
2. It would be better for you not to stay up too late.(表示委婉)
http://cache.baidu.com/c?word=would%2Ccould%2Cshould%2Cmight&url=http%3A//flc%2Ezjgsu%2Eedu%2Ecn/englishonline/ksfd/gcss/Grammar/subjunctive%2Dmood%2Easp&b=14&a=1&user=baidu
拟语气是英文中一特殊的语言现象,主要用于表达与事实相反的陈述,常表达强烈愿望、遗憾、感慨、后悔、责备、规劝等语义,可大致分为三类
一、对现在事实的虚拟
基本形式：If + were /did等过去式…, …would /could /should /might + do
例如：If I were a bird, I would fly to the moon.
(事实上,I'm not a bird, so I will not fly to the moon.)
If she knew who you are, she would go out of joy.
(事实上,She doesn't know, so she still feels joyous.)
I would certainly go if I had the time.
How nice it would be if you could stay a bit longer.
二、对过去事实的虚拟
基本形式：If + had done…, …would /could / should /might + have done
例如：If she had been warned earlier, she wouldn't have broken the rules.
(事实上：She was not warned earlier and she broke the rules.)
If it hadn't rained, the match would have seemed more fascinating.
(事实上：It rained and the match was less fascinating.)
She would have come if we had invited her.
此种虚拟可进行倒装,如：
Had the letter been sent out, it would have ruined our friendship.
= If the letter had been sent out, it would have ruined our friendship.
三、对将来事实的虚拟
基本形式：If + should do…, …would /could /should /might + do; were+to+v（可能性极小）,意思类似汉语中的"万一"
例如：If he should forget the date, I might teach him a good lesson.
(事实上：他不大可能忘记那个日期)
If it should snow this afternoon, we could make a snowman.
(事实上不大可能会下雪)
此种虚拟可倒装,如：
Should my car fail, I would have to return home on foot.
= If my car should fail, I would have to return home on foot.
如果条件从句中包含有were, had, should或could,有时可把if省略掉,但这时要把were, had, should或could放在主语前面（这种结构在口语中较少使用）.例如：
1）Had we made (= If we had made ) adequate preparations, we might have succeeded.
2）Were there no gravity, there would be no air around us.
3）Should it rain tomorrow, the meeting would be put off.
2．用在含蓄条件句中
句子中,假设的情况不用条件从句表示出来,而是用一个介词短语来表示,句子的谓语形式也可遵循表7的规则.这种用法也常见于or, or else, otherwise和but后面的分句中.例如：
1）Without music, the world would be a dull place.
2）But for your help, we would have failed.
3）She would have gone with us, but she didn’t have time.
4）He must have had an accident, or he would have been here then.
3．用在错综时间条件句中
有时候,条件从句表示的动作和主句表示的动作发生的时间并不一致,这时,谓语动词的形式要根据表示的时间来调整,这种句子可称作错综时间条件句.例如：
1）If you hadn’t watched television so late, you wouldn’t be so sleepy now.（此句为时间交错的虚拟条件句,从句表示与过去事实相反的假设,主句则假设与现在情况相反）
2）If you were in better health, we would have allowed you to join them in the work.（从句表示与现在事实相反的假设,主句则表示一种与过去事实相反的设想）
4．用在wish后的宾语从句中
这种句子主要表示愿望,that常省略.若指现在或将来的愿望,从句用过去时；若指过去没有实现的愿望,从句用过去完成时.例如：
1）I wish I had learned to play chess.（与过去事实相反）
2）I wish I remembered his address.（与现在事实相反）
3）I wish he would try again.（对将来情况的假设）
若wish 后的宾语从句中用would,可以表示请求,通常意味着说话人的不快或不满.例如：
1）I wish you would help us.
2）I wish you would stop asking such silly questions.
5．用在if only感叹句中
当表示“但愿”,“要是…就好”时,它的用法和wish基本相同,只是更富有感情色彩,表示一时很难实现的愿望.例如：
1）If only I knew what you wanted.
2）If only you hadn’t told him what I said, everything would have been all right.
6．用在以as if (as though) 引出的方式状语从句中
当表示过去想象中的动作或情况时,用过去完成时；当表示和现在事实相反或对现在情况有所怀疑时,用一般过去时.例如：
1）The old couple loved the orphan girl as if she had been their daughter.
2）It seems as if it were spring.
7．用在某些虚拟的定语从句中
例1：He was the very person I should have been wished to see.
例2：It’s (high) time that we went to class.
9．用以表示说话人的主观愿望或选择,仅用在一些惯用句型的从句中
例1：I’d rather they came tomorrow.
例2：He’d rather I went to Fuzhou by sea.
例3：I’d just as soon you didn’t watch television this evening.
但had rather, would rather, would just as soon, would sooner等惯用语如用在简单句中,其后都用不加to的动词不定式.例如：
1）I’d rather die than give up my principle.
2）I’d just as soon (would sooner) resign than take part in such dishonest deals.
二、用固定的情态动词表示的虚拟语气
1. should +动词原形（有时省略should）
（1）用在动词如advise, command, demand, desire, insist, order, propose, recommend, request, require, suggest, urge等后的宾语从句中.例如：
1）They requested that we (should) send a delegation to their country.
2）She urged that he write and accept the post.
（2）用在it is suggested, it is desired, it is required, it was ordered, it was proposed, it has been decided, it is necessary (essential, imperative, important, desirable) that等引出的主语从句中.例如：
1）It is desired that we (should) get everything ready by tonight.
2）It is necessary that the teacher (should) have a thorough knowledge of the subject he teaches.
（3）用在suggestion, motion, proposal, order, recommendation, plan, idea, requirement等引起的表语从句和同位语从句
例如：He put his coat over the child lest he should catch cold.
4．用在表示比拟的方式状语从句中
例如：They talked as if they had been friends for years.
5．用在表示虚拟情况的定语从句中
例如：It is high time you handed in your test paper.
6．用在某些表示主观愿望的名词从句中
例1：I wish I were as strong as you.
例2：He insisted that we (should) take up the matter at the meeting.
7．用在婉转的请求、建议、批评等句子中
例1：Could you spare me a few minutes?
例2：You might have told me earlier.
例3：Hadn’t you better go and see you dentist about that tooth?
8．用在表示祝愿的简单句中
例1：Long live world peace.
例2：May you succeed.