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出。