There are seven vowels in Greek:
Of these ε and ο are always short, and take about half the time to pronounce as η and ω, which are always long.
A diphthong combines two vowels in one syllable. The second vowel is or ι or υ. The diphthongs are αι, ει, αυ, ευ, ου, ηυ, and, υι. ι written below the line of α, η, and ω is called iota subscript. These are improper diphthongs.