"Those foxes are eating a duck or a goose."

Translation: Ty lišky žerou kachnu nebo husu.

[Helenagvtierrez]

ty lisky zerou a kachna nebo husu. this was my answer and i don't understand the difference it makes if i say kachNU or kachNA

[VladaFu]

It is the accusative case. See the Tips and notes.

And do not put the "a" there.