YOU SAID:

why do this, when that is good.

INTO JAPANESE

それが良いのに、なぜこれを行うのか。

BACK INTO ENGLISH

Why do this if it's good?

INTO JAPANESE

それが良いことならなぜこれを行うのでしょうか？

BACK INTO ENGLISH

Why do this if it's a good thing?

INTO JAPANESE

それが良いことであるなら、なぜこれを行うのでしょうか？

BACK INTO ENGLISH

If it's a good thing, why do this?

INTO JAPANESE

それが良いことなら、なぜそうするのでしょうか？

BACK INTO ENGLISH

If it's a good thing, why do it?

INTO JAPANESE

それが良いことなら、なぜそうするのでしょうか？

BACK INTO ENGLISH

If it's a good thing, why do it?