It's because in English the word "cheese" is uncountable. "A piece of cheese" is grammatically correct.

It shouldn't be, really.

kanapka is the singular nominative case of the word- used as subject of the sentence and in "to" sentences

kanapkę is singular accusative case. - it is most common case for the direct object of verb ( for example jeść=eat, and mieć=have), also after some prepositions.

Kanapka - sandwich; kanapka - little sofa Kanapa - sofa

Sorry, what is the question exactly? You mixed Polish and English and I don't know where you would like to put the 'a'.

"cheese" is uncountable, so is "soup", so you generally don't put "a" in front of them.