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)