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