If it were not for… If it had not been for … but for… 有什么关系和

If it were not for 是对现在的虚拟,意思是：如果不是因为或若不是.如.If it were not for the bad weather now ,we would go outing .
If it had not been fo是对过去的虚拟,意思和上面一样,如 If it had not been for the bad weather yesterday,we would have gone outing .
but for若不是,也用在虚拟句中,如 But for your help,I could not have finished the work on time yesterday.(But for your help= If it had not been for your help)

一If it were not for… 如果不是因为…
这里的if 引导的虚拟语气，表现在的结构从句用if +wereV-ed 主句用 would should might could +v原形
If it were not for the fact that she is ill, she would take part in the match. 如果不是因为她...