"The pigs play in the park."
Translation:La porkoj ludas en la parko.
"La porkoj" is an Esperantist rock band, "ludi" = play, so I read "La porkoj ludas en la parko" and I think: There is a concert in the park....
Grammatically speaking, "en la parko" here is an adverbial, not an accusative. It shows where they play, not what they play. Accusatives answer the question "what"
The n goes on the noun the verb is acting upon. the pigs are not playing the park, they are playing in the park.
More simply put, accusative -n is not used after prepositions (unless it's about direction, which I assume will be covered in later skills)
(However there are exceptions, which can be overlooked at this stage but are worth pointing out. E.g. to denote movement into something.)
I'm used to multiple conjugations of "the" now (el, la, los, las), and coming back to Esperanto is twisting my brain a bit (with only la).