ιη is used as follows (in all other cases -ι_-):—
a. In contracted verbs in the singular, rarely in the dual and plural. -ι_- appears in the dual and plural, rarely in the singular.
b. In liquid verbs in the future active singular: φανοίη-ν for φανεο-ίη-ν. In the dual and plural -ι_-: φανοῖτον, φανοῖμεν for φανεό-ι_-τον, φανεό-ι_-μεν.
c. In the singular of μι-verbs: τιθείην for τιθε-ίη-ν, διδοίην for διδο-ίη-ν, θείην for θε-ίη-ν. Here the modal sign is added to the tense-stem without any thematic vowel. -ι_- is more common in the dual and plural: τιθεῖμεν for τιθέ-ι_-μεν, διδοῖμεν for διδό-ι_-μεν, θεῖτε for θέ-ι_-τε. Verbs in -νυ_μι make their optatives like λύ_ω.
d. In the aorist passive: λυθείην for λυθε-ίη-ν, φανείην for φανε-ίη-ν. In the dual and plural -ι_- is more common: λυθεῖμεν for λυθέ-ι_-μεν, φανεῖτε for φανέ-ι_-τε.
e. In some second perfects, as προεληλυθοίης, and in the second aorist σχοίην from ἔχω (but -σχοῖμι in composition).
N.—In the 3 pl. -ιε- is regular before -ν: λύ_ο-ιε-ν, τιθε-ῖε-ν, λυθε-ῖε-ν.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].