1. Stem vowels are, as with the noun, the vowel endings of the Stem, especially when they are derivatives. They are -i, -u, -a, -e (and also -o in Roots). But the most extended thematic vowel, that which is called theme or thematic vowel, that which exists since IE II and has overshadowed the (older) athematic stems is e/o/e/o. The thematization of stems, so to speak, has relegated the athematic forms especially to the past and to the perfect; most of the old athematics, even those in -a- and -e-, are usually extended with thematics -ie- or -io-.
NOTE. The old thematics were usually remade, but there are some which resisted this trend; as bhero, I bear, do, I give, or i!, go!
The theme vowel has sometimes a meaning, as with -e- and -a-, which can indicate state. There are also some old specializations of meanings, based on oppositions:
a. Thematic stem against athematic:
- An Indicative athematic is opposed to a thematic Subjunctive. The contrary is rare.
- A thematic Present is opposed to an athematic Imperfect, and vice versa.
- Sometimes, the first person singular and plural and the third person plural are thematic, and the rest are athematics.
- It can also be found in the Middle-Active voice opposition.
b. Thematic stem with variants:
- The first person, thematic in lengthened -o.
- Thematic o in the first person singular and plural and third person plural; e in second and third person singular and second plural. There is also an archaic third person plural in e, as in sent, they are.
c. Opposition of thematic stems.
This is obtained with different vowel grades of the root and by the accent position.
2. In the semithematic inflection the athematic forms alternate with thematic ones.
NOTE. The semithematic is for some an innovation of the late IE III which didn't reach some of the dialects; while for others it represents a situation in which the opposition thematic-athematic and the accent shifts of an older system has been forgotten, leaving only some remains.