The stem of the second aorist passive is formed by adding -η- (or -ε-) directly to the verb-stem. Thus, ἐβλάβην
a. -η- appears in the indicative, imperative (except the third plural), and infinitive; -ε- appears in the other moods. -η- is found before a single consonant, -ε- before two consonants or a vowel except in the nom. neut. of the participle.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].