Herbert Weir Smyth, A Greek Grammar for Colleges

a. An o sound always prevails over an a or e sound: ο or ω before or after α, and before η, forms ω. οε and εο form ου (a spurious diphthong, 6). Thus, τι_μάομεν ῀ τι_μῶμεν, αἰδόα ῀ αἰδῶ, ἥρωα ῀ ἥρω, τι_μάω ῀ τι_μῶ, δηλόητε ῀ δηλῶτε; but φιλέομεν ῀ φιλοῦμεν, δηλόετον ῀ δηλοῦτον.

b. When α and ε or η come together the vowel sound that precedes prevails, and we have α_ or η: ὅραε ῀ ὅρα_, τι_μάητε ῀ τί_μᾶτε, ὄρεα ῀ ὄρη.

c. υ rarely contracts: υ ¨ ι ῀ υ_ in ἰχθύ_διον from ἰχθυίδιον small fish; υ ¨ ε strictly never becomes υ_ (273).