"A duck is eating bread."
Translation:Kaczka je chleb.
What noun case is 'chleb' in this sentence? Is it accusative (biernik) as it is the direct object of the sentence?
yes it is accusative. but chleb is masculine inanimate noun and has accusative=nominative.
"jedzą" (with a tail) is 3rd person plural. You have 3rd person singular here, and it is "je".