What happened to the expected -ε- thematic vowel in present active indicative 1 p sg and 3 p pl?

1. What is the origin of the thematic vowel?
2. What are thematic verbs?
3. What is the difference between thematic and athematic stems?

What is the origin of the thematic vowel?

Etymology. In the term thematic vowel, theme refers to the stem of a word. For example, in the Ancient Greek verb τέμνω (témnō) 'cut', tem- is the root, and temn- is the stem or theme for the present tense. Hence, thematic vowel loosely means "stem vowel".

What are thematic verbs?

noun. : a verb whose present tense stem ends in the varying thematic vowel e:o, (as Latin seque, sequo)

What is the difference between thematic and athematic stems?

Generally speaking, verbs that use the thematic vowel (i.e., –ω verbs) are also called THEMATIC VERBS. Likewise, verb tenses that do not use the thematic vowel (i.e., –μι verbs) are called ATHEMATIC VERBS.