14.3 The verbs that we met earlier use the 1st person singular present indicative active ending –μι. These types of verbs, unsurprisingly, are known as –μι verbs. While some of the most common verbs in Greek are -μι verbs, most Greek verbs are –ω verbs, so called because they use the 1st person singular present indicative active ending –ω. Both types of verbs build and parse the same way. They just use somewhat different endings to designate person and number.
All –ω verbs have a sort of buffer sound just before the verb’s ending. This buffer is a vowel sound called the THEMATIC VOWEL (S 377). (An older linguistic term for the base or stem of a word is "theme," hence the name of this vowel). The –μι verbs do not have this vowel. This thematic vowel, usually an ο or ε in Greek, blends with the verb ending in a stable, consistent way, so we learn the thematic vowel and the personal ending together as one combined personal ending.
The good news is that the thematic vowel keeps the verb ending stable! Remember that some –μι verbs change vowel lengths or make other changes when a –σ– is added to a verb stem. These types of changes do not happen as often with –ω verbs because of the thematic vowel.
Verbs that use the thematic vowel (e.g., –ω verbs) are called THEMATIC VERBS. Likewise, verbs that do not use the thematic vowel (e.g., –μι verbs) are called ATHEMATIC VERBS.