zamenjivati

1. to replace; to stand in for; to substitute
   Dok sam na odmoru, koleginica me zamenjuje. — While I'm on holiday, a colleague stands in for me.
   Šećer često zamenjujem medom. — I often replace sugar with honey.
2. (+ s + instr.) to mistake; to confuse
   Ljudi ga stalno zamenjuju s bratom. — People keep mistaking him for his brother.
3. to exchange; to swap
   U ovoj prodavnici bez problema zamenjuju neispravnu robu. — This shop exchanges faulty goods without any fuss.

«Zamenjivati + acc. + instr.» means to replace with; «zamenjivati koga s kim» means to confuse with.