- φιλέ-ετε → φιλεῖτε "you (pl.) love"
- νόος → νοῦς "mind"
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, and coexist beside forms with the other grade:
- λείπω "I leave" (e-grade: genuine diphthong) — λέ-λοιπα "I have left" (o-grade)
- ἐλεύ(θ)σομαι "I will come" (e-grade) — Homeric εἰλ-ήλουθα "I have come" (o-grade)
Early in the history of Greek, the diphthong versions of ει and ου would have been pronounced as [ei̯, ou̯], and 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 the vowels are pronounced short, as [i, u].
Long e or 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 this was the vowel of contraction. Though later forms of Doric contracted to ει, ου, throughout the history of Doric, compensatory lengthening used η, ω.
- Herbert Weir Smyth, Greek Grammar, par. 25: diphthongs
- Smyth, par. 6: genuine and spurious ei, ou
- Smyth, par. 113: τι̯, θι̯ → σ(σ)
- Smyth, par. 221: -ya in short-a feminine
- ἔρχομαι. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project
- Smyth, par. 36: vowel-grades
- Smyth, par. 59 note 5: Severer Doric εε → η; οο, εο → ω
- Smyth, par. 37 note 2: Doric compensatory lengthening