Possession with the Genitive
To say “X of Y” or “Y’s X”, put the possessor in the genitive and place it after the thing possessed: το σπίτι του δασκάλου (“the house of the teacher / the teacher’s house”). The possessed noun keeps the case its own role demands; the possessor is always genitive. This is the everyday way Greek marks belonging.
The rule
To say “X of Y” or “Y's X”, Greek puts the possessor in the genitive and places it after the thing possessed: το σπίτι του δασκάλου (“the teacher's house”), το όνομα του παιδιού, η πόρτα του σπιτιού. The two halves move independently and it pays to keep them apart. The possessed noun takes whatever case its own role demands - nominative as a subject (το σπίτι του δασκάλου είναι μεγάλο), accusative as an object (βλέπω το σπίτι του δασκάλου). The possessor is genitive always, whatever is happening around it. And the order is the mirror image of the English teacher's house: possessed first, possessor second.