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1)时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
a . 同现在事实相反的假设。
句型 ： 条件从句 主句
一般过去时 should ( would ) +动词原形
If they were here , they would help you .
b . 表示于过去事实相反的假设。
句型： 条件从句 主句
过去完成时 should ( would ) have + 过去分词
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 .
含义： 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 .
含义： He was ill and missed many lessons , so he did not make greater progress .
c . 表示对将来的假想
句型： 条件从句 主句
一般过去时 should + 动词原形
were + 不定式 would + 动词原形
should + 动词原形
If you succeeded , everything would be all right .
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