虛擬語氣是一種動詞形式，表示說話人的一種愿望，假設(shè)，懷疑，猜測， 建議等含義，虛擬語氣所表示的含義不是客觀存在的事實。在條件句中的應(yīng)用，條件句可分為兩類，一類為真實條件句，一類為非真實條件句。非真實條件句表示的是假設(shè)的或?qū)嶋H可能性不大的情況，故采用虛擬語氣。
一、虛擬語氣在從句中的使用
虛擬語氣即表達(dá)不可能發(fā)生或與預(yù)期相反的情況。首先我們應(yīng)熟悉它在三種時態(tài)里的使用情況，這是正確使用虛擬語氣的基礎(chǔ)。這三種時態(tài)是：現(xiàn)在；過去；將來。
1、表示現(xiàn)在的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
現(xiàn) 在
did
would + do
If I were a bird, I would fly to you. 如果我是只鳥，我就會飛到你身邊去。
If it were not for their help, we should be in a very difficult position.
如果不是他們的幫忙，我們就會處于一個非常困難的境地。
2、表示過去的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
過 去
had + done
would + have + done
If we had started earlier, we should not have missed the train.
如果我們早一點動身的話，就不會誤車了。
If she hadn’t been ill, she might have come. 如果她不是病了，可能會來的
3、表示將來的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
將 來
should + do
were to + do
would + do
If Professor Li should have time tomorrow, we could ask him some questions.
如果李教授明天有空的話，我們可以問他點問題。
If there should be no air, there would be no living things.
如果沒有空氣，就不會有生物。
二、錯綜時間的虛擬句
通常情況下,在非真實條件句中主句和從句的謂語動詞所指時間是一致的, 但有時也可能指不同的時間,這時要根據(jù)上下文的意思采用不同的謂語動詞形式.
If I were you, I would have taken his advice. (從句指現(xiàn)在, 主句指過去)。
換成我是你，我就會聽從他的建議。
If I had taken my raincoat with me this morning, I would not be wet now. (從句指過去,主句指現(xiàn)在)。如果我今天早上帶了雨衣，現(xiàn)在就不會被淋濕了。
If we hadn’t been working hard in the past few years, things wouldn’t be going so smoothly.
（從句指過去，主句指現(xiàn)在）。如果過去幾年里不努力工作，現(xiàn)在的情況就不會如此順利了。
If the weather had been more favorable, the crops would be growing still better.（從句指過去，主句指現(xiàn)在）。如果當(dāng)時的天氣更好點，現(xiàn)在的莊稼就會長得更好。
If we had not got everything ready by now, we should be having a more terrible time tomorrow. （從句指現(xiàn)在，主句指將來）
如果現(xiàn)在一切還沒準(zhǔn)備好，明天情況就更糟了。
三、含蓄條件句
有時假設(shè)的情況并不以條件從句表示出來,二是暗含在上下文中,比如通過介詞短語來表示。這些介詞常是：but for, without, otherwis等。如：
But for your advice, I could not have done it so successfully.
如果不是聽了你的建議，我就不會做的如此成功。
The change could not have taken place without the open-door policy.
沒有開放的政策，就不會有如此大的變化。
I was so busy then, Otherwise, I would have told him the answer.
我當(dāng)時很忙，否則，我就會把答案告訴他的。
一、虛擬語氣在從句中的使用
虛擬語氣即表達(dá)不可能發(fā)生或與預(yù)期相反的情況。首先我們應(yīng)熟悉它在三種時態(tài)里的使用情況，這是正確使用虛擬語氣的基礎(chǔ)。這三種時態(tài)是：現(xiàn)在；過去；將來。
1、表示現(xiàn)在的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
現(xiàn) 在
did
would + do
If I were a bird, I would fly to you. 如果我是只鳥，我就會飛到你身邊去。
If it were not for their help, we should be in a very difficult position.
如果不是他們的幫忙，我們就會處于一個非常困難的境地。
2、表示過去的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
過 去
had + done
would + have + done
If we had started earlier, we should not have missed the train.
如果我們早一點動身的話，就不會誤車了。
If she hadn’t been ill, she might have come. 如果她不是病了，可能會來的
3、表示將來的虛擬式
時 態(tài)
從 句 動 詞
主 句 動 詞
將 來
should + do
were to + do
would + do
If Professor Li should have time tomorrow, we could ask him some questions.
如果李教授明天有空的話，我們可以問他點問題。
If there should be no air, there would be no living things.
如果沒有空氣，就不會有生物。
二、錯綜時間的虛擬句
通常情況下,在非真實條件句中主句和從句的謂語動詞所指時間是一致的, 但有時也可能指不同的時間,這時要根據(jù)上下文的意思采用不同的謂語動詞形式.
If I were you, I would have taken his advice. (從句指現(xiàn)在, 主句指過去)。
換成我是你，我就會聽從他的建議。
If I had taken my raincoat with me this morning, I would not be wet now. (從句指過去,主句指現(xiàn)在)。如果我今天早上帶了雨衣，現(xiàn)在就不會被淋濕了。
If we hadn’t been working hard in the past few years, things wouldn’t be going so smoothly.
（從句指過去，主句指現(xiàn)在）。如果過去幾年里不努力工作，現(xiàn)在的情況就不會如此順利了。
If the weather had been more favorable, the crops would be growing still better.（從句指過去，主句指現(xiàn)在）。如果當(dāng)時的天氣更好點，現(xiàn)在的莊稼就會長得更好。
If we had not got everything ready by now, we should be having a more terrible time tomorrow. （從句指現(xiàn)在，主句指將來）
如果現(xiàn)在一切還沒準(zhǔn)備好，明天情況就更糟了。
三、含蓄條件句
有時假設(shè)的情況并不以條件從句表示出來,二是暗含在上下文中,比如通過介詞短語來表示。這些介詞常是：but for, without, otherwis等。如：
But for your advice, I could not have done it so successfully.
如果不是聽了你的建議，我就不會做的如此成功。
The change could not have taken place without the open-door policy.
沒有開放的政策，就不會有如此大的變化。
I was so busy then, Otherwise, I would have told him the answer.
我當(dāng)時很忙，否則，我就會把答案告訴他的。