It's because in English the word "cheese" is uncountable. "A piece of cheese" is grammatically correct.
What is the difference between 'kanapka' and 'kanapke (with that hook thing on the e?)
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.
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.