"A black horse is eating bread."
Translation:Czarny koń je chleb.
Czarnego is the genitive and m. anim. accusative form of black, czarny is the m. inanim. accusative and nominative form for all genders. In this case, the nominative case is being used because koń is the subject.
Ok, so if the horse was the direct object it would take the genitive form of black instead in that case?
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)