YOU SAID:
I don't know whether the weather will be nice tomorrow.
INTO JAPANESE
明日天気が良くなるかどうかわかりません。
BACK INTO ENGLISH
I do not know if the weather will improve tomorrow.
INTO JAPANESE
明日天気が良くなるかどうかわかりません。
BACK INTO ENGLISH
I do not know if the weather will improve tomorrow.
You've done this before, haven't you.