早知如此何必当初用英语怎么说？？速度啊！！！！！！！

why did you do that If you would have known this?

觉得好怪。。。

Had I know it, I wouldn't have done it。

No need to cry over the spilled milk.
也可以说是“覆水难收”。

Coulda Woulda Shoulda

If I'd known then what was going to happen, I wouldn't have done as I did.
早知今日，悔不当初。
来源： dj.iciba.com