YOU SAID:

there's black stuff all over my pants

INTO JAPANESE

私のズボンの中の黒いものがあります。

BACK INTO ENGLISH

Black stuff in my pants.

INTO JAPANESE

私のズボンに黒のもの。

BACK INTO ENGLISH

My pants are black.

INTO JAPANESE

私のズボンは黒です。

BACK INTO ENGLISH

My pants are black.