Dative is the one that can answer the question "to whom" To whom did you give the money? To Ivan - Ивану Genitive is the one that say that something belongs to someone. This one pretty much exists in english as well. If you would use "'s", at the end of noun in English or "of" noun, it's most likely you'd use genitive in russian as well Ivan's car - Car of Ivan Машина Ивана You use nominative when you need answer to question "who" or "what" Who is it? It's Ivan Это Иван. Instrumental you use, when you are using something, or doing something with someone. I speak with Ivan. Я говорю с Иваном