English:
The letters a, e, i, o, and u are vowels. All other letters are consonants.
Greek:
The letters α, ε, η, ι, ο, υ, and ω are vowels.
Hebrew:
See page 5 in KHW or these videos and/or flashcards or this. cf. half vowel, full vowel, long vowel, short vowel.
