Stems in οι, with nominative in -ώ, turn ι into unwritten y (
N. πειθώ. G. πειθοῦς (πειθό-ος). D. πειθοῖ (πειθό-ι). A. πειθώ (πειθό-α). V. πειθοῖ. Dual and plural are wanting.
a. A stronger form of the stem is ωι, seen in the earlier form of the nominative (Σαπφῴ, Λητῴ). The accusative has the accent of the nominative.
b. When dual and plural occur, they are of the second declension: nom. λεχοί (late) from λεχώ
c. ἡ εἰκών
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].