This is the 4177th most frequent Polish word.
chłopiec
Boy.
The word 'chłopiec' is used as the subject of the sentence, indicating a male child who is performing the action.
Chłopiec bawi się na placu zabaw.
The boy is playing on the playground.
Here, 'chłopca' is the genitive case of 'chłopiec' used to indicate possession or relation.
Mama kupiła książkę dla chłopca.
Mom bought a book for the boy.
The word 'chłopca' in the accusative case of 'chłopiec', specifies whom the observer saw.