"A black horse is eating bread."
Translation:Czarny koń je chleb.
The adjective takes the same form as the noun it describes, so if the direct object is in the genitive case it would take a genitive form and if it's in the accusative case it will take the accusative form, e.g.:
Widzę czarne konie ('I see black horses', accusative case)
Nie widzę czarnych koni ('I don't see black horses', genitive case)