Question

If it were not for… If it had not been for … but for… 有什么关系和区别，请高手详细讲解，谢谢

Answer 1

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 +were\V-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. 如果不是因为她病了，她肯定会参加比赛，她现在病了。
二If it had not been for …如果没有…
结构︰if it had not been for＋名词／子句，主词＋would／should 等＋原形动词…
此句型意为“若非（有）…”。此类句构专门用于表示与过去事实相反的<假设语气>。
例子：If it had not been for his help, I should have failed. 如果没有他的帮忙，我应该已经失败。
三but for… 但对于…
英语中 for sb/sth 与 to sb/sth不同，for sb\sth 含意思是对于某人或者某物有益 有帮助
to sb \sth 有属于的意思　并且to 这个介词有方向性
What's too much for me, just right for two, but nothing at all for three?
什么对我一个人太多，两个人正好，三个人就什么也没有了？
Of the world you are only a person.But for me you're my world.
对于这个世界来说，你只是一个微不足道的人，但是对于我来说，你是我的全部.

希望能解开你的困惑~~

Answer 2

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)