_____,I'd have told you.
a, if I have known it b,if i have had known it c,had i known it d,should i know it 答案选c，不知道为什么
虚拟语气
与现在相反 从句IF + 主语+动词过去式
主句 主语+WOULD/SHOULD+ 动词原形
与过去相反 从句 IF+主语+动词的过去完成式
主句 主语+SHOULD+HAVE+过去分词
与将来相反 从句A 谓语用动词典过去式
B谓语用WERE TO +动词原形
C谓语用SHOULD+动词原形
主句 主语+/WOULD/SHOULD+动词原形
从句如省略IF，用倒装句式“WERE/HAD/SHOULD+主语
eg。
Were I in school again ,I would work harder。 与现在事实相反
原句 If I were in school…
Had you been here earlier,you would have seen him。
与过去事实相反
原句 If you had been here…
Should there be a meeting tomorrow,I would come。 与将来相反
原句If there should be a meeting…
题目中这句话是对过去事实的相反，如果我已经知道了，就告诉你了
If I had known it,I would have told you。
省略IF，所以选C
。
