The quantity of most syllables is usually apparent. Thus, syllables
a. with η, ω, or a diphthong, are long.
b. with ε, ο, before a vowel or a single consonant, are short.
c. with ε, ο, before two consonants, or a double consonant, are long.
d. with α, ι, υ, before two consonants, or a double consonant, are long.
N.—But syllables with ε, ο, or α, ι, υ before a stop and a liquid may be short ( cross145). Cp. also 147 c.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].