The word of the day is…
ἐγώ, μου or ἐμοῦ, μοι or ἐμοί, με or ἐμέ
Greek, egō, mou/emou, moi/emoi, me/eme, 1. personal pronoun, “I, me” (with forms in the singular nominative, genitive, dative and accusative, respectively)
In my experience, the enclitic versions of the singular first person pronoun are more common. This is definitely the case in Hebrews, on which I am basing my current WOTDs (the phrase υἱός μου εἶ σύ from 1:5 is to blame for today’s WOTD).
My earlier post on the related plural first-person pronoun should probably be combined with this one, as the forms are certainly related through a lengthened pre-vowel and the consonant -μ-; maybe I will produce a post in the future that is the full “declension” of the personal pronouns, to be placed under Greek study rather than Word of the Day.
Just as a note, the first personal pronoun has a much richer representation in writing than just what is listed here. Above is what is found in Attic, and the forms are a bit different in the inflection of Herodotus and Homer. Not sure about Koine, which is a progression from Attic. It may only use the enclitic forms above (where there is a choice) – which may be why that is what we find in, say, Hebrews.