等你知道我为什么难过，你就会明白我为什么如此对你！英文翻译怎么说呢

When you know why I am unhappy,you will know why I did that to you.
可能是这样翻译吧

When you know why I'm sad with, you will understand why I treated you like this!

When you know why I am so sad, you will know why I traded you like that.

Not until you know why I am so sad will you understand why I did that to you.

you will understand why i treat you in this way until you know why i am so sad.