przeciwniku

In a sentence

• Przeciwniku, nie jesteś jeszcze pokonany.

  Opponent, you are not defeated yet.

  Here 'przeciwniku' is used to directly address someone who is an opponent in a vocative case.

• Widzę mojego przeciwnika na końcu sali.

  I see my opponent at the end of the hall.

  In this sentence, 'przeciwnika' is in the accusative case, used as the object of the verb 'widzę' (I see).

• Rozmawiamy o naszym przeciwniku w grze.

  We are talking about our opponent in the game.

  Here 'przeciwniku' is in the locative case, following the preposition 'o' (about) to indicate the topic of discussion.