Hiatus is allowed in certain cases.
1. In epic poetry: a. After ι and υ: ἄξονι ἀμφίς, σύ ἐσσι.
b. After a long final syllable having the rhythmic accent: μοι ἐθέλουσα ([macrdot]˘˘[macrdot]˘).
c. When a long final syllable is shortened before an initial vowel (
d. When the concurrent vowels are separated by the caesura; often after the fourth foot: ἀλλ' ἄγ' ἐμῶν ὀχέων ἐπιβήσεο, ὄφρα ἴδηαι; very often between the short syllables of the third foot (the feminine caesura): as, ἀλλ' ἀκέουσα κάθησο, ἐμῷ δ' ἐπιπείθεο μύ_θῳ; rarely after the first foot: αὐτὰρ ὁ ἔγνω A 333.
e. Where ϝ has been lost.
2. In Attic poetry hiatus is allowable, as in 1 c, and after τί
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].