In Attic-Ionic Greek the dipthong ει may be either genuine (gen.) or spurious (sp.)
The two spurious dipthongs ει and ου are digraphs (two letters representing a single sound) that started to be used around the late fifth century BC to represent long versions of the short vowels ε and ο since Greek at that time had no specific way of representing those two long vowels.
There’s a very good explanation of this in chapter 1 of the Cambridge Grammar of Classical Greek (CGCG) which, if you read it about a half-dozen times, may eventually make sense. At least that’s how long it took for me to understand it
Thanks a lot, Mitch!
Tico: I didn’t get the vowel contraction table from a book, I found it in a professor’s online course notes a couple months ago (I don’'t remember the references unfortunately). I would be happy to post the table in a Word doc in the Resources section, but I don’t know the posting procedure (or whether there would be any interest).
FWIW vowel contraction in ancient Greek made no sense to me until I learned about vowel height/backness/roundedness and how Greek vowels were positioned on the vowel triangle. All this is covered in chapter 1 of CGCG, and once you know the basics of Greek vowel phonology you can (almost) derive the table of vowel contractions from first principles vs. using (ugh) rote memorization.