虛擬語氣，又稱假設語氣，是謂語動詞的一種形式，表示說話人敘述的內容與事實相反，在現實中并不存在，或實現的可能性很小。
If I were a bird, I could fly in the air.
A 虛擬語氣的三大種類
與現在事實相反
if條件句的謂語 were\ did 主句的謂語 would (could/should/might) + do
If I were you, I should go and try.
I would go if they treated me like a slave.
與過去事實相反
if條件句的謂語 had done
主句的謂語 would (could/should/might) + have done
If it had not rained this morning, I should have gone shopping.
與將來事實相反
if條件句的謂語 were\ did\ should do\ were to do
主句的謂語 would (could/should/might) + do
If he were here tomorrow, I would speak to him.
If she had time, she could help me.
If it should rain, the crops could be saved.
If I were to do the work, I should do it in a different way.
B 省略if的條件句 （用倒裝結構）
Were it necessary, I might go without delay. (= If it were necessary,...)
Had you taken my advice, you wouldn't have failed in the exam.
(= If you had taken my advice,...)
C 錯綜條件句
虛擬條件句中的主句和從句涉及的動作發生的時間不一致。
If you hadn't watched TV yesterday, you wouldn't be so sleepy now.
If you hesitated this moment, you might suffer in the future.
D 含蓄條件句
非真實條件句中的條件從句有時不表達出來，只暗含在上下文中，這種句子叫做含蓄條件句。
He must have been there, or he never could know the place so well.
Such mistakes could have been avoided.
E 虛擬語氣在從句中的用法
主句中的動詞、名詞、形容詞如表示要求、建議、命令或愿望等含義，與其相關的名詞性從句(主語從句、賓語從句、表語從句和同位語從句)中的謂語動詞要用虛擬語氣。
It is suggested that each student (should) sing a song in English.
The workers demanded that their wages (should) be raised by 10 percent.
My advice is that you (should) practice speaking English as often as possible.
在It is (high) time (that) ...結構中，定語從句的謂語動詞用過去式，表示“到某人該做某事的時間了”。
It is（high） time that the children went to bed.
The teacher treats the pupil as if he were her own child. （方式狀語從句）
I felt as though we had known each other for years. （方式狀語從句）
She emphasized it again and again, lest he (should) forget. （目的狀語從句）
Though he (should) fail, there would still be hope. （讓步狀語從句）
F 其它句型中的虛擬語氣
1, if only...(如果…就好了) If only you had told me the truth before.
2, would rather （寧愿） I would rather I hadn't seen that film yesterday
