Hi Ania Marysia. I already hasten to explain to you the difference between these "goodbye" words in Polish? "Pa" is used when we say goodbye to someone for a while or we say goodbye to someone to see them at another time of the day or the next day. "Do zobaczenia" we use to say goodbye to someone and to see them in the near future. "Cześć" we use to say goodbye to someone for a short or longer time, but we also use when greeting someone. "Dobranoc" we use when saying goodbye to someone in the evening to see them the next day. "Żegnać się" is the phrase we use when we say goodbye to someone forever. "Żegna" is the third person singular form "żegnać się", e.g. "Marek żegna się z Ewą i nigdy się już nie zobaczą". I hope that my answer has helped you.