I had never kown it ___ she told me.请问横线处为何填until而不填before

efore只是陈述以前的事实,用until则着重后来的变化情况.这里句子意思是直到她告诉我我才知道这件事.具体辨析如下：
until/not until是指：直到什么时候为止...其中非延续性动词用后者,延续性动词用前者.
例句：Not until my father came back did I fell asleep.
I was awake until my father came back.
注意：倒装时倒装主句,从句语序不变.改为强调句时基本语序不变.
before在...之前,常用在时间状语从句中.
常用句型：It is +( )+before...
注意：前后时态要一致,遵从主将从现原则.
祝楼主进步!
要是答案还满意的话,记得采纳哦,O(∩_∩)O谢谢~!
翻译：1...就...It was not long before he turned against his country.不久他就背叛了他的祖国.
2...才...It was a long time before he came to himself.很久之后他才恢复意识.

until
过去完成时，必须用until，而不是before, 这是语法规定

I had never kown，用的是过去完成时，用Until，表示过去我一直不知道，直到他告诉我，从语法来讲必须用它，可在实际生活交流中也无所谓，两者也可以互换，呵呵