1. 表示本打算做什么，结果未能实现，体现遗憾惋惜的心情。常用短语：mean to，intend to，plan to。
例如：I had meant to go on Sunday but have stayed on.
I intended to have helped him, but I was very busy at that time.
2.表示一种相反的条件下会出现什么样的结果。常用or，otherwise引起虚拟语气。
例如：I ran all the way, or I should have been late for school.
I lost your telephone number, otherwise I would have rung you up long before.
3. 为了简化语言，句中暗含一种条件，理解为“要是有，要是没有，要不是”等。常用介语with，without，but for，if only引起虚拟语气。
例如：Without animals and plants, human being would not exist.
Without air(If there were no air ) there would be no living.
But for the storm, we should have had a much happier time.
要不是暴风雨，我们本可以玩得更快乐。
If only he had more courage.
4.表示说话者的一种假设，理解为“好象，比如”。常用连词if，as if，as though等引起虚拟语气。
If I were you, I would go to night school.
They talked as if they had been friends for many years.
5.表示说话者一种委婉的建议和提议。常用It’s time接宾语从句， I’d rather后跟从句。
例如：It’s time you went to school.
It’s high time we did our homework.
I’d rather you went there tomorrow.
6.表示说话者对一种情况的客观、委婉的判断。常用It is important /strange/natural/necessary that…句型来表达。
例如：It’s important that every member (should) inform himself of these rulers.
It’s necessary that he (should) be sent there at once.
It’s very strange that she (should) have left without saying goodbye.
7.表示说话者对过去、现在、将来的一种愿望、希望，常用wish后跟宾语从句。
例如：I wish I knew the answer to the question.
How I wish I were a bird! I wish you would come soon.
8 .在表坚持、命令、建议、要求的动词(或其ed形式)和名词后跟的.名词性从句中，体现说话者的一种委婉的提议、建议等。常见动词：insist，suggest，order，propose，command，demand，reguest，desire，require，urge (后跟宾语从句)。
常见名词：insistence，suggestion，proposition，requirement (其中order，demand，request，desire，command既是动词，又是名词)后跟的名词性从句中要用should +v，should也可以省掉。
例如：I suggest that we (should) set off at one.
The students insisted that they (should) have more.
