First of all, σε δίνω is incorrect as δίνω κάτι ΣΕ κάποιον (I give something TO someone) would be the full phrase. In other words, use με, σε, τον, etc. when there's no preposition like "to" or "for", and when there is use μου, σου, του, etc. In answer to your main question, you use the Nominative for talking about the subject, you use the Genitive for possession and INDIRECT objects (i.e. taking a preposition), and the Accusative for talking about the DIRECT object. For example: Ο Μιχάλης τρέχει. (Michael is the subject, i.e. BEFORE the verb) Ο Γιώργος βλέπει τον Μιχάλη. (Michael is the direct object, i.e. AFTER the verb, no preposition) Ο Γιώγρος μιλάει στον Μιχάλη / ο Γιώργος μιλάει του Μιχάλη (rare) / ο Γιώργος του μιλάει. (Michael is the indirect object, i.e. AFTER verb WITH a preposition) Το βιβλίο είναι του Μιχάλη. (Michael owns the book). Hope this helps. There are other uses of each case that I thought I best not get into here. If you'd like to know, however, just let me know! ;)