YOU SAID:

One thing, I don't know why. No matter how hard I try.

INTO JAPANESE

ひとつ、理由がわかりません。どんなに頑張っても。

BACK INTO ENGLISH

I don't know the reason. No matter how hard you try.

INTO JAPANESE

理由はわかりません。どんなに頑張ってもダメ。

BACK INTO ENGLISH

I do not know why. No matter how hard you try

INTO JAPANESE

どうしてか分かりません。どんなに頑張っても

BACK INTO ENGLISH

I do not know why. No matter how hard you try