Loss of unstressed initial vowels

An unstressed non-low word-initial vowel in earlier Greek, that is, any vowel (or diphthong) except for α (long or short), was lost in late Byzantine / early Medieval Greek.

Understanding this late Byzantine sound change helps students make sense of many otherwise puzzling forms, notably the absence of the past indicative augment in first and second person plural forms, e.g. γράψαμε γrápsame ‘we wrote’,  γράψατε γrápsate ‘you all wrote’ (versus its retention when stressed, as in 1sg έγραψα éγrapsa ‘I wrote’); many if not most regional dialects preserve the unstressed augment.

But this change also affected nouns and verbs and even prefixes, such as the following:

ντύνομαι, ντυνω < ενδύω

μπαίνω < εμπαἰνω

ντρέπομαι < εντρέπομαι

λιακάδα < ηλιακάδα

κτίριο < οικτήριον

σπίτι < οσπίτιν < οσπίτιον < (Latin) hospitium

μάτι < ομμάτιον (Aristotle)

ξε- < εξ-ε- (-ε- added on presumably via reanalysis of augment in past tense forms, e.g. εκ-φράζω with past tense εξ-έφρασα reanalyzed as εξέ-φρασα, probably aided by plural forms where in verbs without the prefix, there would be no augment (due to the loss of unstressed initial vowels)).

Many strange forms make sense when the initial vowel is replaced; see Horrocks p. 207