虛擬語氣是一種動(dòng)詞形式，表示說話人的一種愿望，假設(shè)，懷疑，猜測(cè)， 建議等含義，虛擬語氣所表示的含義不是客觀存在的事實(shí)。在條件句中的應(yīng)用，條件句可分為兩類，一類為真實(shí)條件句，一類為非真實(shí)條件句。非真實(shí)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(shí)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(shí)態(tài)是：現(xiàn)在；過去；將來。
1、表示現(xiàn)在的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 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(huì)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(huì)處于一個(gè)非常困難的境地。
2、表示過去的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 詞
過 去had donewould have done
If we had started earlier, we should not have missed the train.
如果我們?cè)缫稽c(diǎn)動(dòng)身的話，就不會(huì)誤車了。
If she hadn’t been ill, she might have come. 如果她不是病了，可能會(huì)來的
3、表示將來的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 詞
將 來should do
were to dowould 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(diǎn)問題。
If there should be no air, there would be no living things.
如果沒有空氣，就不會(huì)有生物。
6、過去進(jìn)行時(shí)
過去進(jìn)行時(shí)是由助動(dòng)詞be的過去式加上現(xiàn)在分詞構(gòu)成的，主要表示過去某個(gè)時(shí)刻正在進(jìn)行的動(dòng)作。
At that time she was working as an assistant in a physics labatory.
那時(shí)她在一家物理實(shí)驗(yàn)室做助手。
They took us to see the chemical fibre mill that they were building.
他們帶我們?nèi)⒂^了他們正在修建的化纖廠。
7、過去完成時(shí)
過去完成時(shí)由助動(dòng)詞had加動(dòng)詞的過去分詞構(gòu)成，主要表示過去某時(shí)刻前業(yè)已完成的動(dòng)作或存在的狀態(tài)，其實(shí)就是指過去的過去。使用時(shí)往往要要在從句中或有一個(gè)明確的過去時(shí) 間狀語。
Before daybreak they had covered half the distance. 拂曉前他們已經(jīng)走完了大半路程。
No sooner had the rain stopped than they set out again. 雨一停他們就又上路了。
He had not learned any English before he came to the university.
上大學(xué)前他一點(diǎn)英語沒學(xué)過。
8、一般過去將來時(shí)
一般過去將來時(shí)由助動(dòng)詞would加動(dòng)詞原形構(gòu)成，主要表示從過去某一時(shí) 間角度看以后將要發(fā)生的動(dòng)作。含這個(gè)時(shí)態(tài)的句子常帶一個(gè)表示“過去某個(gè)時(shí) 間點(diǎn)”的狀語。這個(gè)狀語或是一個(gè)短語，或是一個(gè)句子。這個(gè)時(shí)態(tài)常用于：賓語從句或間接引語中。例如：
When I thought about it, I wondered what their reaction would be.
當(dāng)我考慮這件事時(shí)，我想知道他們的反應(yīng)是什么。
She told me that she would go on trip to Europe the next day.
她告訴我，她第二天要去歐洲旅行。
Whenever he had time, he would help his mother with some housework.
無論他什么時(shí) 間有空,他總是幫他媽媽干點(diǎn)家務(wù)活。
No matter how difficult the work was , he would keep on doing it until he accomplished it . 不管工作有多難，他總會(huì)堅(jiān)持不懈地把它干完。
9、一般將來完成時(shí)
一般將來完成時(shí)由shall (will) have 過去分詞構(gòu)成。主要表示在將來某時(shí)業(yè)已發(fā)生的動(dòng)作。如：
After we finish this text, we’ll have learned twenty texts.
這課課文結(jié)束后，我們就學(xué)了二十課課文了。
By the next month, I’ll have finished my task. 到下個(gè)月，我將完成任務(wù)。
They will have hit the year’s target by the end of October.
到十月底他們就可完成全年的指標(biāo)了。
10、現(xiàn)在完成進(jìn)行時(shí)
現(xiàn)在完成進(jìn)行時(shí)由have（has）been加動(dòng)詞的現(xiàn)在分詞構(gòu)成，表示發(fā)生在過去的某個(gè)動(dòng)作延續(xù)到現(xiàn)在或剛剛完成，或許還要繼續(xù)延續(xù)。這種時(shí)態(tài)側(cè)重于這個(gè)動(dòng)作的連續(xù)性，或者說不間斷性。例如：
I’ve been waiting for you for the whole morning. 整個(gè)上午我一直在等你。
It has been raining for three days. 雨連續(xù)下了三天。
Where have you been? 你去哪兒啦？
She has been busy preparing the exhibition since we met last time.
自我們上次見面以來，她就一直忙著籌備那個(gè)展覽。
11、過去完成進(jìn)行時(shí)
過去完成進(jìn)行時(shí)由had been加現(xiàn)在分詞構(gòu)成，表示過去某一時(shí)刻之前開始的動(dòng)作或狀態(tài)一直延續(xù)到過去某一時(shí)刻。例如：
It had been raining cats and dogs for over a week and the downpour had caused landslides in many places. 傾盆大雨下了一周，在許多地區(qū)引發(fā)了山崩。
At last they gor the telegram they had been expecting.
后他們收到了他們一直盼望的電報(bào)。
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(shí)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(shí)態(tài)是：現(xiàn)在；過去；將來。
1、表示現(xiàn)在的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 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(huì)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(huì)處于一個(gè)非常困難的境地。
2、表示過去的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 詞
過 去had donewould have done
If we had started earlier, we should not have missed the train.
如果我們?cè)缫稽c(diǎn)動(dòng)身的話，就不會(huì)誤車了。
If she hadn’t been ill, she might have come. 如果她不是病了，可能會(huì)來的
3、表示將來的虛擬式
時(shí) 態(tài)從 句 動(dòng) 詞主 句 動(dòng) 詞
將 來should do
were to dowould 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(diǎn)問題。
If there should be no air, there would be no living things.
如果沒有空氣，就不會(huì)有生物。
6、過去進(jìn)行時(shí)
過去進(jìn)行時(shí)是由助動(dòng)詞be的過去式加上現(xiàn)在分詞構(gòu)成的，主要表示過去某個(gè)時(shí)刻正在進(jìn)行的動(dòng)作。
At that time she was working as an assistant in a physics labatory.
那時(shí)她在一家物理實(shí)驗(yàn)室做助手。
They took us to see the chemical fibre mill that they were building.
他們帶我們?nèi)⒂^了他們正在修建的化纖廠。
7、過去完成時(shí)
過去完成時(shí)由助動(dòng)詞had加動(dòng)詞的過去分詞構(gòu)成，主要表示過去某時(shí)刻前業(yè)已完成的動(dòng)作或存在的狀態(tài)，其實(shí)就是指過去的過去。使用時(shí)往往要要在從句中或有一個(gè)明確的過去時(shí) 間狀語。
Before daybreak they had covered half the distance. 拂曉前他們已經(jīng)走完了大半路程。
No sooner had the rain stopped than they set out again. 雨一停他們就又上路了。
He had not learned any English before he came to the university.
上大學(xué)前他一點(diǎn)英語沒學(xué)過。
8、一般過去將來時(shí)
一般過去將來時(shí)由助動(dòng)詞would加動(dòng)詞原形構(gòu)成，主要表示從過去某一時(shí) 間角度看以后將要發(fā)生的動(dòng)作。含這個(gè)時(shí)態(tài)的句子常帶一個(gè)表示“過去某個(gè)時(shí) 間點(diǎn)”的狀語。這個(gè)狀語或是一個(gè)短語，或是一個(gè)句子。這個(gè)時(shí)態(tài)常用于：賓語從句或間接引語中。例如：
When I thought about it, I wondered what their reaction would be.
當(dāng)我考慮這件事時(shí)，我想知道他們的反應(yīng)是什么。
She told me that she would go on trip to Europe the next day.
她告訴我，她第二天要去歐洲旅行。
Whenever he had time, he would help his mother with some housework.
無論他什么時(shí) 間有空,他總是幫他媽媽干點(diǎn)家務(wù)活。
No matter how difficult the work was , he would keep on doing it until he accomplished it . 不管工作有多難，他總會(huì)堅(jiān)持不懈地把它干完。
9、一般將來完成時(shí)
一般將來完成時(shí)由shall (will) have 過去分詞構(gòu)成。主要表示在將來某時(shí)業(yè)已發(fā)生的動(dòng)作。如：
After we finish this text, we’ll have learned twenty texts.
這課課文結(jié)束后，我們就學(xué)了二十課課文了。
By the next month, I’ll have finished my task. 到下個(gè)月，我將完成任務(wù)。
They will have hit the year’s target by the end of October.
到十月底他們就可完成全年的指標(biāo)了。
10、現(xiàn)在完成進(jìn)行時(shí)
現(xiàn)在完成進(jìn)行時(shí)由have（has）been加動(dòng)詞的現(xiàn)在分詞構(gòu)成，表示發(fā)生在過去的某個(gè)動(dòng)作延續(xù)到現(xiàn)在或剛剛完成，或許還要繼續(xù)延續(xù)。這種時(shí)態(tài)側(cè)重于這個(gè)動(dòng)作的連續(xù)性，或者說不間斷性。例如：
I’ve been waiting for you for the whole morning. 整個(gè)上午我一直在等你。
It has been raining for three days. 雨連續(xù)下了三天。
Where have you been? 你去哪兒啦？
She has been busy preparing the exhibition since we met last time.
自我們上次見面以來，她就一直忙著籌備那個(gè)展覽。
11、過去完成進(jìn)行時(shí)
過去完成進(jìn)行時(shí)由had been加現(xiàn)在分詞構(gòu)成，表示過去某一時(shí)刻之前開始的動(dòng)作或狀態(tài)一直延續(xù)到過去某一時(shí)刻。例如：
It had been raining cats and dogs for over a week and the downpour had caused landslides in many places. 傾盆大雨下了一周，在許多地區(qū)引發(fā)了山崩。
At last they gor the telegram they had been expecting.
后他們收到了他們一直盼望的電報(bào)。