YOU SAID:

I do not understand why you would do such a thing

INTO JAPANESE

なぜそんなことをするのか分かりません

BACK INTO ENGLISH

I don't know why you do that

INTO JAPANESE

なぜそうするのか分かりません

BACK INTO ENGLISH

I don't know why

INTO JAPANESE

どうしてか分からない

BACK INTO ENGLISH

I don't know why.

INTO JAPANESE

どうしてか分からない

BACK INTO ENGLISH

I don't know why.