Vowel Gradation (35, 36).—Verbs of the first class show a variation between a strong grade (or two strong grades) and a weak grade. The weak grades, ι, υ, α, appear especially in the second aorist and second passive systems; the corresponding strong grades, ει (οι), ευ (ου), η (ω), appear usually in the other systems (οι, ου, ω, in the second perfect).
a. Expulsion of a short vowel between consonants (so-called syncope cross493) produces a weak form of the stem of the same grade as ι, υ, α ( cross36). Cp. γί-γν-ομαι
b. α is the weak form of η (α_), as in τήκω ἐτάκην; and of ε, when ε has λ, μ, ν, ρ before or after it, as in τρέπω, ἐτράπην ( cross479).
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].