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時態：可以表示過去，現在和將來的情況。它的基本特點是時態退後。
a.同現在事實相反的假設。
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.表示於過去事實相反的假設。
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.
c.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.
虛擬語氣1.概念
虛擬語氣用來表示說話人的主觀願望或假想，所說的是一個條件，不一定是事實，或與事實相反。
2.在條件句中的'應用
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的或實際可能性不大的情況，故採用虛擬語氣。
真實條件句
真實條件句用於陳述語氣，假設的情況可能發生，其中if是“如果”的意思。
時態關係
句型：條件從句+主句 一般現在時：shall/will + 動詞原形
If he comes, he will bring his violin.
典型例題
The volleyball match will be put off if it ___.
A. will rain B. rains C. rained D. is rained
答案B。真實條件句主句為將來時，從句用一般現在時。
注意：
1) 在真實條件句中，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，而直接用一般現在時的動詞形式。
