The verb-stem agrees with that of the perfect middle herein:
a. Vowel verbs lengthen the final vowel of the verb-stem, as τε-τί_μη-μαι, ἐ-τι_μή-θην. On verbs which do not lengthen their final vowel, see cross488.
b. Liquid stems of one syllable change ε to α, as τέ-τα-μαι, ἐ-τά-θην (τείνω
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].