In order to form the nominative, genitive and accusative the following questions are asked.

Nominative: Who?/What?
Ποιός είναι στο τηλέφωνο; (Who is on the phone?/ Who is speaking?)
Η Μαρία! (Maria!)

Genitive: Whose?
Τίνος είναι το κόκκινο αυτοκίνητο; (Whose car is this?)
Το κόκκινο αυτοκίνητο είναι του Κώστα. (The red car belongs to Kostas.)

Accusative: Whom?/What?
Τι δώρο σου έκανε στα γενέθλιά σου; (What did he give to you for your birthday?)
Ένα πανέμρφο μαντήλι. (A beautiful scarf.)

There is no dative case existing in Greek.
Instead there is the vocative case.