"Kawa" is Nominative form. "Kawę" is Accusative form, for example: "Piję [kogo? co?] kawę"
(I am drinking [whom? what?] coffee).
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Kawa - is nominative singular form - one that is shown in dictionaries, written on labels, receipts and menu, it is a form that is used when the word is used as subject of the sentence.
Kawę - (with "tail", there is no such word as "KAWE") is accusative singular form. It is used as direct object of most verbs in positive sentence.
For example "Piję kawę"= I am drinking coffee, Poproszę kawę- Coffee, please (literally "I will ask for coffee")