"A duck is eating a cookie."
Translation:Kaczka je ciasteczko.
The basic word in nominative case is actually „ciasteczko”, but here we used it in accusative (which is identical to nominative in neuter gender).
„Ciasteczka” would be genitive singular, nominative plural, accusative plural or (rare for this word) vocative plural.
„Je” is for third person singular (he/she/it) and „jedzą” for third person plural (they).
American English Cookies British English biscuits Polish herbatniki American English and British English cake Polish ciasteczko Why do you keep using cookie for ciasteczko?
I'm not sure where you have seen those Polish translations before, but allow me to clarify.
"Herbatnik" is a kind of simple cookie, typically in a form of thin wafer. As the name suggests, it might be fitting for tea tasting.
"Ciasteczko" is your typical cookie with a flavor (chocolate for example). Though perhaps you may classify "herbatnik" as a kind of "ciasteczko".
At the moment I can't think of a situation where a cake would become "ciasteczko". If it's a really small cake, then it would probably be "ciastko". A celebratory kind of cake (like birthday or wedding cake) is "tort".