1. 表示本打算做什麼，結果未能實現，體現遺憾惋惜的心情。常用短語：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.表示一種相反的條件下會出現什麼樣的結果。常用or，otherwise引起虛擬語氣。
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. 爲了簡化語言，句中暗含一種條件，理解爲“要是有，要是沒有，要不是”等。常用介語with，without，but for，if only引起虛擬語氣。
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.
要不是暴風雨，我們本可以玩得更快樂。
If only he had more courage.
4.表示說話者的一種假設，理解爲“好象，比如”。常用連詞if，as if，as though等引起虛擬語氣。
If I were you, I would go to night school.
They talked as if they had been friends for many years.
5.表示說話者一種委婉的建議和提議。常用It’s time接賓語從句， I’d rather後跟從句。
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.表示說話者對一種情況的客觀、委婉的判斷。常用It is important /strange/natural/necessary that…句型來表達。
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.表示說話者對過去、現在、將來的一種願望、希望，常用wish後跟賓語從句。
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 .在表堅持、命令、建議、要求的動詞(或其ed形式)和名詞後跟的.名詞性從句中，體現說話者的一種委婉的提議、建議等。常見動詞：insist，suggest，order，propose，command，demand，reguest，desire，require，urge (後跟賓語從句)。
常見名詞：insistence，suggestion，proposition，requirement (其中order，demand，request，desire，command既是動詞，又是名詞)後跟的名詞性從句中要用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.