語氣是動詞的一種形式，表示說話人的看法和態(tài)度。英語有三種語氣，即陳述語氣、祈使語氣和虛擬語氣。陳述語氣主要用于陳述句和疑問句，用于陳述事實、提出看法或問題等;祈使語氣用于祈使句，表示請求、命令或警告等;虛擬語氣則表示假想、推測或主觀愿望，而非客觀存在的事實。如：Don’t be late next time(祈使語氣)。If I were you, I would not go. (虛擬語氣)I wish I had a lot of money. (虛擬語氣)。
在虛擬語氣中，含有非真實條件句的復(fù)合句使用最為普遍，學(xué)習(xí)時要特別注意從句和主句謂語動詞(因動作發(fā)生時間不同而具有)的幾種不同組合形式。另外，一些特定的英語句型或表達(dá)中常需要使用虛擬語氣，也應(yīng)是我們在學(xué)習(xí)時特別關(guān)注的地方。
考點一：虛擬語氣的三種基本類型
1.與現(xiàn)在事實相反：①條件從句謂語：動詞過去式(be 一般用were)
②主句謂語：would/should/could/might +動詞原形
例如：If I saw him now, I would be very happy.
If I were here now, we could ask him about it.
2.與過去事實相反：①條件從句謂語：had+過去分詞
②主句謂語：would/should/could/might + have+過去分詞
例如：If I had seen him then, I would have been very happy.
If I had been here then, we could have asked him about it.
3.與將來事實相反：(1)條件從句謂語：①動詞過去式②should+動詞原形③were to+動詞原形
(2)主句謂語：would/should/could/might+動詞原形
例如：If I were/should be/were to be you tomorrow, I shouldn’t quarrel with him.
Even if 引導(dǎo)的讓步狀語從句，其謂語形式與if 虛擬條件句相同。
Nothing could have saved the wounded man even if he had been treated without delay.
注意：
1.虛擬條件句中謂語動詞變化助記口訣：碰到虛擬條件句，時態(tài)向前移一位。假設(shè)現(xiàn)在某情況，從句應(yīng)用過去時。假設(shè)過去某情況，應(yīng)用過去完成時。
2.上述主句謂語中，would表示結(jié)果，might表示可能性，could 表示能力、允許或可能性，should主要用于第一人稱后。
3.條件從句的謂語動詞為be時，不管其主語為單數(shù)還是復(fù)數(shù)通常都用were。
4.與將來事實相反的虛擬語氣中，當(dāng)條件從句謂語動詞使用should +動詞原形時(表示可能性極小，常譯為萬一)，主句謂語動詞除了would/should/could/might +動詞原形這樣的虛擬語氣外，也可用陳述語氣或祈使語氣
例如：If I should see him, I will tell him.
If it should rain tomorrow, don’t except me.
考點二：(時間)混合虛擬語氣
條件從句的動作和主句的動作發(fā)生的時間不一致(如一個是過去發(fā)生的，一個是現(xiàn)在發(fā)生的)。這時主從句動詞的形式要根據(jù)時間來調(diào)整，使用各自的謂語動詞。
例如： Tom would be alive today if the doctor had come sooner last night.
If he had followed th doctor’s advice then, he would be quite all right now.
考點三： 倒裝的虛擬條件句(were, had, should)
If 條件句中有were, had(助動詞)和should時，可使用倒裝結(jié)構(gòu)，但if必須省略。
If I had seen him then, I would have been very happy. =Had I see him then, I would have been very happy.
If I should have time, I would go with you. =should I have time, I would go with you.
若條件從句為否定句，否定詞not應(yīng)置于主語之后，而不能與were, should, had 等縮成weren’t, shouldn’t, hadn’t而置于句首。
If I were not so busy, I would go with you. =Were I not so busy, I would go with you.
考點四：含蓄條件句的虛擬語氣(without，with，but，for，or，otherwise或上下文)
用without(=but for)，or (else)，otherwise暗示if從句
例如：but for his help, we wouldn’t have made such great progress.
He must have missed the train, or he would have arrived here then.
當(dāng)上下文或上下句中的語境暗示的時候，省略條件句，避免重復(fù)。
It was not a matter of money. Even a very poor man would have done the same, I think.
They helped me a lot. I don’t know what I would have done without their help.
考點五：虛擬條件句兩個常用的固定句式(if it weren’t for…和if it hadn’t been for…)
If it weren’t for…和if it hadn’t been for…是兩個很正常的虛擬語氣句型，其意為若不是(有);要不是有。
其倒裝形式是：Were it not for…和Had it not been for…
此句型可與but for, without “要不是’(后加名詞，句子謂語動詞常與虛擬語氣)等替換。
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