虛擬語氣是一種動詞形式，表示說話人的一種愿望，假設，懷疑，建議等含義，虛擬語氣所表示的含義不是客觀存在的。
一．在非真實條件句中
虛擬條件句（if從句） 主句
與現(xiàn)在事實
相反 主語 + 主語 + would/could/might/ should + do
與過去事實
相反 主語 + 主語 + would/could/might/should + have done
與將來事實
相反 ①主語 + did
②主語 +were to do
③主語 + should do 主語 + would/could/might/should + do
1 If I were you, I would buy it. 2 If she knew English, she would ask me for help now.
3 If you had got up earlier, you would have caught the train yesterday.
注意：錯綜時間條件句：從句和主句發(fā)生的時間不一致時，動詞的形式要分別根據(jù)它所表示的不同時間作相應的調整。 5 If you had followed my advice (聽從)yesterday, you would be better now.
6 If they had studied hard in the past, they would do it easily now.
二 省略if的情況: 虛擬條件句中含were , should, had 時，可省略if,把這幾個詞置于句首。
1 Were I you, I wouldn’t do that. （be） 2. Had you come earlier, you would have seen him.
3. Should it rain tomorrow, I would stay at home.
三 含蓄條件句 :使用with, without, but, or,or else, otherwise， but for(= if it were not for…與現(xiàn)在事實相反；if it had not been for…與過去事實相反)等代條件句。
1 We couldn’t have succeeded without your help.(沒有你的幫助)
2 But for the rain（要不是） we would have finished the work.
3 He telephoned to inform me of your birthday. or (=If he hadn’t telephoned to inform me of your birthday)I would have known nothing about it.
但是： If it were not for the fact that she ____sing, I would invite her to the party. C
A. couldn’t B. shouldn’t C. can’t D. mightn’t
四 常用虛擬語氣的句型
(一) 在下列句型中，常用“should + 動詞原形”且should可以省略：
1 表示要求，建議，命令，提議，意愿等的主語從句，賓語從句，表語從句和同位語從句中。
一堅持insist; 二命令order, command; 三建議suggest, advise,propose; 四要求demand,require,request,ask等后。
注意：當suggest作__________講，insist作_________講，后邊that從句不用虛擬語氣。
2 在It is + 形容詞(necessary, natural, strange, etc. ) + that從句中。
在It is/was a pity（a shame, no wonder, etc. ） + that從句中。
1）We all agreed to his suggestion（that）we (should abandon the plan. 放棄那個計劃.
2) It is suggested that the meeting should be put off next week.（被推遲）
3) It’s necessary/important/natural that we_______________________每天打掃房間.
但是1) Mike insisted that he had never stolen anything. （從未偷過）
2) The look on his face suggested that he was angry.
但是： It is（high/about）time（that）sb.did sth /should do sth
Eg: It is high time we started / should start. 動身出發(fā)
(二) 用動詞的過去式表示與現(xiàn)在事實（或將來事實）相反，用動詞的過去完成式表示與過去事實相反的句型。
1)．wish后that引導的賓語從句中。(表示對將來的設想，用could/would/might+v.)
2)．a(chǎn)s if（though）引導的表語從句或方式狀語從句中。
3)．would rather后的賓語從句中。
4)．If only引導的感嘆句中。
1 I wish I _______________________. 和你一樣聰明。
2 I wish he ______________________ tomorrow. 再次嘗試
3 He looks as if/though he ____________________. 他是一個藝術家
4 I’d rather you _________________ there yesterday. 不去
5 Look at the terrible situation I am in! If only I _______________ (follow) your advice.