"Are you eating a sandwich?"
Well, maybe it's a huge sandwich, some Guinness World Records one. I agree that it's a lot less probable variant, but it's a grammatically correct translation.
However, it's now deleted from the "best solutions" and only "Jesz" should be suggested to the learners, "Jecie" will just simply be accepted if someone writes it.
This is a very broad topic, so I'd advice you to take a look here and find posts about cases: https://www.duolingo.com/comment/16296174
But in short words:
Nominative is the basic, dictionary form. So it will mostly be used for the subject of the sentence, or as "Y" in sentences like "X to Y" (X is already in Nominative as it's the subject) or "X jest Y" when Y is an adjective.
Accusative is the case used for the direct object of the sentence.
To understand why it is Kanapkę and not Kanapka, i explain abit about Biernik or Accusative case. in Polish Language, or most of Slavic languages, The verbs are the most important factor.In English we do not conjugate Nouns and even Verbs.but in Slavic Languages both of them are conjugated in their manner. Accusative form is used in below situations: 1-for Verbs which are smenatically positive, like: Kochać, Lubić, Mieć, Widzieć, Znać,Jeść, Oglondać,Pić, Płasać, and Tonczyć. so any verb which show you an ability or powerful act will be in Accusative case. 2-when we have prepositions like below, our case will be Accusative: Przez, na, o, po, pod, nad, za, przed, między, ponad, poza and w. so what happens for the ends of the nouns and adjectives which are Accusative? you have to focus that in Polish 4 Genders exist. 1.Masculine and Alive 2.Masculine and dead 3.Feminine 4.Neutral so if a noun and it's adjective are in this cases you have to do these following changes: 1->for Masculine and alive: Na przekład: Mam PrzystojnEGO ChłopakA i have a handsome boy. so the noun end is A and the adjective EGO. 2->this is what you have asked here that why Kanapka is wrong and Kanapkę is correct? Na przekład: Mam ładną dziewczynę. you see? ę is ended for feminine, because Kanapka is feminine. 3->Mam CzerwonE WinO 4->Mam NowY Komputer-