As you may know, in Greek we have pronouns that are inflected (nominative, genitive, accusative). Let us summarise a few points for you!
Usually, in nominative case, we do not use the subject pronoun since we can understand who is the subject by the ending of the verb: «Θέλω να πάω στο σούπερ μάρκετ» (thélo na páo sto súper márket) = I want to go to the supermarket (we don’t need to use «εγώ»).
*We can use the pronoun for emphatic reasons: «Εγώ θέλω να πάω στο σούπερ μάρκετ, όχι εσύ» (eghó thélo na páo sto súper market, óhi esí) that is “I want to go to the supermarket, not you”.
| Ονομαστική (onomastikí) = nominative | |
| εγώ (eghó) = I εσύ (esí) = you αυτός (aftós) / αυτή (aftí) / αυτό (aftó) = he / she / it εμείς (emís) = we εσείς (esís) = you αυτοί (aftí) / αυτές (aftés) / αυτά (aftá) = they (all genders) | 
We use the accusative case of the pronouns when they substitute direct objects: «Η Άννα ξέρει τη Μαρία. Την ξέρει.» (I Άna xéri ti María. Tin xéri) = Anna knows Maria. She knows her.
Lastly, we use the genitive case of the pronouns when they substitute indirect objects: «Μίλησες στον Γιάννη; Ναι, του μίλησα.» (mílises ston Ghiáni? Ne, tu mílisa) = Did you talk to John? Yes, I talked to him.
| Αιτιατική (etiatikí) = accusative | Γενική (ghenikí) = genitive | 
| με (me) = me σε (se) = you τον (ton) / τη(ν) [ti(n)] / το (to) = him / her / it μας (mas) = us σας (sas) = you τους (tus) / τις (tis) / τα (ta) = them (all genders) | μου (mu) = to me σου (su) = to you του (tu) / της (tis) / του (tu) = to him / her / it μας (mas) = to us σας (sas) = to you τους (tus) = to them (all genders) | 
*These weak pronouns both in accusative and in genitive immediately precede the verb.
