Lengthening of Short Final Vowel.—Verb-stems ending in a short vowel generally lengthen that vowel before the tense-suffix in all tenses (except the present and imperfect) formed from them. Here α (except after ε, ι, and ρ) and ε become η, ο become ω.
a. Note ἀκροά_σομαι, ἠκροα_σάμην, etc., from ἀκροάομαι
b. Verb-stems adding ε or ο ( cross486), and stems apparently receiving a short final vowel by metathesis ( cross128), lengthen the short final vowel, as βούλομαι (βουλ-)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].