YOU SAID:

there's a black stuff under my fingernail

INTO JAPANESE

私の爪の下に黒いものがあります。

BACK INTO ENGLISH

Under my fingernails black.

INTO JAPANESE

私の爪は黒い。

BACK INTO ENGLISH

My nails are black.

INTO JAPANESE

私の爪は黒です。

BACK INTO ENGLISH

My nails are black.