Thematic Vowel.—The thematic, or variable, vowel appears at the end of the tense-stems in the present, imperfect, and second aorist active and
middle of ω-verbs, and in all futures and future perfects. The thematic vowel in the indicative is ο before μ or ν (and in the optative of the tenses mentioned); elsewhere it is ε. Thus, λυ_όε-, λιπόε-, λυ_σόε-, λυθησόε-, λελυ_σόε-; λύ_ο-ι_-μι. In the subjunctive it is ώη.
a. Attic inscriptions have both -εσθων and -οσθων in the imperative.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].