Nouns that have the -n ending are in accusative. For example: ĝi estas kato - it is a cat mi vidas katoN - I see a cat; esperanto estas lingvo - Esperanto is a language mi parolas esperantoN - I speak Esperanto In the first example, "mi" is a subject and "kato" is an object. For instance, in an English sentence "he sees him", "he" is a subject and "him" is an object. Therefore, "him" is an accusative of "he". You don't say "he sees he". It's ultimately the same in Esperanto, as in many other languages, just that those languages go further and extend the use of accusatives to nouns and adjectives, not just pronouns.