Learn Hardcore Polish: Widzę go w parku.

The ending -ę marks the first-person singular present tense of the verb widzieć (“to see”). Because Polish verbs carry person and number endings, the subject pronoun ja (“I”) is understood from -ę and is usually left out unless you need to emphasize I.

Why is go used here instead of on or jego?

On is the nominative form (“he”), and jego is the genitive form (“of him” or colloquially sometimes as object). Since “him” is the direct object of widzę, Polish requires the accusative case. The short accusative form of on is go, so Widzę go correctly means “I see him.”

What case is parku, and why is it used after w?

Park changes to parku in the locative case. Prepositions denoting location like w (“in”) govern the locative. Thus w parku literally means “in the park.”

Why do we use w with parku? Could we use na instead?