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if引导的条件从句，分为真实条件语句和非真实/假设/虚拟条件语句（也就是常说的虚拟语气？）
“虚拟“ 条件从句表示的一定是与真实情况相反的, 可以是跟过去的真实情况“相反” 的，也可以是跟现在/将来真实情况相反的假设。
虚拟条件从句和主句的谓语形式一定是过去形式
主句的谓语助动词一定是： should/would/could/might
从句的谓语形式 + 主句的谓语形式：
与现在情况相反的假设： did / were , should... do : example: if i were you, i would marry her.
与过去情况相反的假设： had done, should... have done: example: if i had not stopped the thief, he would have run away.
与将来情况相反的假设： did/ were to do, should... do： example: if you were to succeed, everything would be all right.
那么， 区分是真实的（通常所说的）条件句还是假设条件句，主要根据日常常识和说话的上下文来分析。
另外，真实的条件句，一般要遵循“主将从现”的语法
虚拟条件句，除了用if引导的从句外，还有一种含蓄条件句， 即没有if引导，但通过上下文，或一些短语,
可以come to a conclusion所说情况表示一种假设的， 也应该使用虚拟语气，如：
without air, there would be no life.
in the old days he would have argued.
but for(要不是） the rain, we should have had a pleasant journey.
现在分词/过去分词短语/otherwise等，都可能表示含蓄条件句， 如：Given more attention, the tree could have grown better.
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原文地址：http://www.cnblogs.com/bkylee/p/4871929.html
