In general, spurious ει, ου contracts from ε, ο + ε, ο, ει, ου. The specific rules are more complex.
By contrast, true diphthongs are e or o placed before i or u. Some come from e-grade of ablaut + i, or o-grade + u, co-existing beside forms with the other grade:
- λείπω "I leave" (e-grade: genuine diphthong) — λέ-λοιπα "I have left" (o-grade)
- *eleútʰ-somai → ἐλεύσομαι "I will come" (e-grade) — Homeric εἰλ-ήλουθα "I have come" (o-grade)
- Proto-Greek *akouyō → ἀκούω "I hear"
Early in the history of Greek, the diphthong versions of ει and ου were pronounced as [ei̯, ou̯], the long vowel versions as [eː, oː]. By the Classical period, the diphthong and long vowel had merged in pronunciation and were both pronounced as long monophthongs [eː, oː].
By the time of Koine Greek, ει and ου had shifted to [iː, uː]. (The shift of a Greek vowel to i is called iotacism.) In Modern Greek, distinctive vowel length has been lost, and all vowels are pronounced short: [i, u].
Long e and o existed in two forms in Attic-Ionic: ει, ου and η, ω (ē, ō). In earlier Severer Doric, by contrast, only η, ω counted as a long vowel, and it was the vowel of contraction. In later forms of Doric, it contracted to ει, ου. Throughout the history of Doric, compensatory lengthening resulted in η, ω.
- Smyth. A Greek grammar for colleges. § 25: diphthongs.
- Smyth. A Greek grammar for colleges. § 6: genuine and spurious ei, ou.
- Smyth. A Greek grammar for colleges. § 113: τι̯, θι̯ → σ(σ).
- Smyth. A Greek grammar for colleges. § 221: -ya in short-a feminine.
- ἔρχομαι. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
- Smyth. A Greek grammar for colleges. § 36: vowel-grades.
- Smyth. A Greek grammar for colleges. § 59.5: Severer Doric εε → η; οο, εο → ω.
- Smyth. A Greek grammar for colleges. § 37.2: Doric compensatory lengthening.