Verbs in -αω, -εω, -οω contract the final α, ε, ο of the verb-stem with the thematic vowel -όε (-ώη) in the present and imperfect tenses. Thus, τι_μάω τι_μῶ, ποιέω ποιῶ, δηλόω δηλῶ; ἐτί_μαον ἐτί_μων, ἐποίεον ἐποίουν, ἐδήλοον ἐδήλουν. The rules of contraction are given in 4955; the paradigms, p. 120.
a. Open forms of -εω verbs occur in the lyric parts of tragedy.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].