1. 虛擬語氣

　　1） 概念

　　虛擬語氣用來表示說話人的主觀愿望或假想，所說的是一個條件，不一定是事實(shí)，或與事實(shí)相反。
　　2） 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。

　　1.1 真實(shí)條件句

　　真實(shí)條件句用于陳述語氣，假設(shè)的情況可能發(fā)生，其中 if 是如果的意思。
 
　　時態(tài)關(guān)系

　　句型： 條件從句　　　 主句

　　一般現(xiàn)在時　　 shall/will + 動詞原形

　　If he comes, he will bring his violin.

　　典型例題

    1.2 表示對將來的假想

　　句型：　 條件從句　　　　　　 主句

　　一般過去時　　　　　 should+ 動詞原形

　　were+ 不定式　　　　 would + 動詞原形

　　should+ 動詞原形

　　If you succeeded, everything would be all right.

　　If you should succeed, everything would be all right.

　　If you were to succeed, everything would be all right.

　　1.3 混合條件句

　　主句與從句的動作發(fā)生在不同的時間，這時主，從句謂語動詞的虛擬語氣形式因時間不同而不同，這叫做混合條件句。

　　If you had asked him yesterday, you would know what to do now. （從句與過去事實(shí)相反，主句與現(xiàn)在事實(shí)相反。）

　　If it had rained last night (過去), it would be very cold today (現(xiàn)在).

　　1.4 虛擬條件句的倒裝，虛擬條件句的從句部分如果含有were, should, 或had, 可將if省略，再把were, should或had 移到從句句首，實(shí)行倒裝。

　　Were they here now, they could help us. =If they were here now, they could help us.

　　Had you come earlier, you would have met him =If you had come earlier, you would have met him.

　　Should it rain, the crops would be saved. =Were it to rain, the crops would be saved.

　　注意：在虛擬語氣的從句中，動詞’be’的過去時態(tài)一律用"were"，不用was， 即在從句中be用were代替。

　　If I were you, I would go to look for him. 如果我是你，就會去找他。
 
　　If he were here, everything would be all right. 如果他在這兒，一切都會好的。

　　典型例題

　　_____ to do the work, I should do it some other day. A. If were I　　 B. I were　C. Were I　D. Was I

　　答案C. 在虛擬條件狀語中如果有were, should, had這三個詞，通常將if省略，主語提前, 變成 were, should, had +主語的形式。但要注意，在虛擬條件狀語從句中，省略連詞的倒裝形式的句首不能用動詞的縮略形式。如我們可說 Were I not to do., 而不能說 Weren’t I to do.

　　The volleyball match will be put off if it ___. A.　will rain　 B. rains　 C. rained　 D. is rained

　　答案B。真實(shí)條件句主句為將來時，從句用一般現(xiàn)在時。
 
　　注意：1） 在真實(shí)條件句中，主句不能用be going to表示將來，該用shall, will. (錯) If you leave now, you are never going to regret it. (對) If you leave now, you will never regret it.

　　2） 表示真理時，主句謂語動詞便不用shall (will) +動詞原形，而直接用一般現(xiàn)在時的動詞形式。

　　2.1 非真實(shí)條件句

　　1）時態(tài)：可以表示過去，現(xiàn)在和將來的情況。它的基本特點(diǎn)是時態(tài)退后。
 
　　a.　 同現(xiàn)在事實(shí)相反的假設(shè)。

　　句型 ： 條件從句　　　　　主句

　　一般過去時　　　should( would) +動詞原形 If they were here, they would help you.

　　b.　 表示于過去事實(shí)相反的假設(shè)。
 
　　句型： 條件從句　　　　　 主句

　　過去完成時　　　 should(would) have+ 過去分詞

　　If she had worked harder, she would have succeeded.

　　The rice would not have been burnt if you had been more careful.

　　If my lawyer had been here last Saturday, he would have prevented me from going.
 
　　If he had come yesterday, I should / would have told him about it. 含義：He did not come yesterday, so I did not tell him about it.

　　If he had not been ill and missed many classes, he would have made greater progress. 含義： He was ill and missed many lessons, so he did not make greater progress.