"The woman is eating a cookie."
Translation:Kobieta je ciasteczko.
ciasteczko is nominative and accusative singular cuasteczka is singular genitive , plural noinative accusative and vocative
Polish people know which case should be used, so we know if it is singular or plural
here kobieta je ciasteczko is singular. Kobieta je ciasteczka is plural (eat needs accusative)
But if the sentence needs genitive, like does not eat
kobieta nie je ciastaczka (singular), kobieta nie je ciasteczek (plural).
there are few nouns that have the same case singular and plural the same, so then you have to find out from context, if it is a midsummer night's dream or midsummer nights' dream