if引导条件状语从句的时候,我们分为两种情况,一种是真实的条件,一种是假设,叫作非真实条件状语从句
当IF引导真实条件状语从句,就要遵循我们所说的主将从现的原则,即主句用一般将来时,从句用一般现在时,比如 If all goes according to plan,the first concert will be Tuesday evening.这个就是真实的.
当IF引导非真实条件状语从句时,就要用虚拟语气表达.
1.if + 一般过去时,与现在事实相反,if i were a bird ,i would fly out of the school.
2.if + should/could/would +do,与将来事实相反,if i would finish the job tomorrow,i will call you.
3.if + had done ,与过去事实相反,if i had asked you how to do it ,i would not make such a fool mistake.
怎么区别,就是看从句的时态,在根据语境判断.