YOU SAID:

i have a bad hair day

INTO JAPANESE

私は髪の毛が悪い日がある

BACK INTO ENGLISH

I have bad hair one day.

INTO JAPANESE

いつか髪の毛が悪い。

BACK INTO ENGLISH

Someday my hair is bad.

INTO JAPANESE

いつか私の髪が悪い。

BACK INTO ENGLISH

Someday my hair will be bad.

INTO JAPANESE

いつか髪の毛が悪くなるだろう。

BACK INTO ENGLISH

Someday my hair will get worse.

INTO JAPANESE

いつか私の髪はひどくなるだろう。

BACK INTO ENGLISH

My hair will be terrible someday.

INTO JAPANESE

私の髪はいつかひどいものになるだろう。

BACK INTO ENGLISH

My hair will be terrible someday.