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1.if结构（非真实条件句——表示的是假设的或实际可能性不大的情况） 与..事实相反 If从句 主句 过去 Had done Would* have done 现在Were/did Would* do 将来Should do/were/were to do Would* do 例句：If there hadn’t been much rain in spring, we would have had a good harvest now. 【过去】 If he smoked less, his cough might be well soon. 【现在】 If the lecturer should be late, you would have to make a speech first. 【将来】 备注： （1）上表中’*’，would都可转换为should、could、might。 （2) 如果为时间错综句的话，左右两栏可互相搭配，排列组合。 例句： He would still be alive today if he hadn’t taken that drug. 【与现在事实相反】 【与过去事实相反】 (3) if可转换为其他e799bee5baa6e79fa5e98193e4b893e5b19e335形式 例句：Without your advice, our meeting wouldn’t have been so successfully. 【过去】 （=>可转换为：If there hadn’t been your advice, we …） Your notes are almost illegible (难以辨认的). Notes typed out would be a lot easier to read. 【将来】 (=> 可转换为：If the notes had been typed out, it would be a lot easier to read.) I should have gone to see Dr. Smith and he might have cured me of the disease.【过去】 (=> 可转换为：If he have gone to see Dr. Smith, he ..)
如果从句copy和主句都表示过去的，那从句用had done,主句用would ,could,might,should+have done
如：If you had taken my advice,you would not have been late for the meeting yesterday.如果从句表示过去的，但主句表示现在的，从句用had done,主句用would ,could,might,should+do,如：If you had taken my advice,you would not be late for the meeting now.这种一般都有个时间，如果没有表示时间的词也可以从上下文看zd出。