Had known it earlier,I would not have gone back!

原句是If I had known it earlier,I would not have gone back!(与过去事实相反,虚拟语气)这里从句与主句主语一致,省略了I和if,是正确的句式.

Had known it earlier, I would not have gone back!
如果听起来有点别扭
如果我说这句话的话 我会说
if i had know it earlier, i wouldn't/would not have gone back!
听起来顺口点

有
前后时态错了

有 肯定！
因为 go不能用于完成时的阿！
因改为have known it earlier, i would went back!

没有错， had known是过去完成时，整个句子是由if引导的虚拟条件句，只不过省略了IF，将had 倒装而已。
三楼的句子错了，would是情态动词，后面绝对是跟动词原形。

没