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.
Well done, yes, well done!