If you’ve learned Greek, you know that the present active indicative paradigm for λυω is:
You also learned that the ε and the ο are not a part of the “true” person-number endings of the verb.
(nb: I haven’t bolded the ω and ου because I don’t want to overwhelm you by telling you that more is involved in the formation of those forms. So…just disregard that for now).
Have you ever wondered why we have ει instead of ε for λυεις and λυει? I, for one, just accepted it as being a part of the person number suffix endings. Well, it’s a bit more complicated than that.
Let’s take the third person singular λυει for example. It comes from the verb λυω, which means that the stem of this verb is λυ. But it’s “true” ending isn’t ει. It’s σι. Whence comes σι?
Well, the theme vowel (ε, ο), which comes between the stem and the ending would be, in this example, an ε. This is according to the rule that says ο is the theme vowel when before μ or ν, and an ε before any other letter.
That gives us the following form:
λυ + ε + σι
But Greek doesn’t like having an intervocalic (between two vowels) sigma. Thus, the sigma drops out:
λυ + ε – σ + ι
That leaves us with:
λυ + ε + ι
And, according to the rules of contraction, the epsilon contracts with the iota to form the diphthong ει, giving us the final form:
Stay tuned to find an explanation for why we have ω in the first singular and ουσιν for the third plural.