Allen and Greenough's New Latin Grammar, section 174

Heavy Construction

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Present Stem.

174.

The parent (Indo-European) speech from which Latin comes had two main classes of verbs: -

1. Thematic Verbs, in which a so-called thematic vowel (eo, in Latin iu) appeared between the root and the personal ending: as, leg-i-tis (for leg-e-tes), leg-u-nt (for leg-o-nti). [1][Cf. -- , -o-; Doric -o-.]
2. Athematic Verbs, in which the personal endings were added directly to the root: as, es-t, es-tis (root ES)[2][Cf. -, - (see p. 83, note).], da-mus (dó, root DA), fer-t (feró, root FER).
Of the Athematic Verbs few survive in Latin, and these are counted as irregular, except such as have been forced into one of the four ``regular'' conjugations. Even the irregular verbs have admitted many forms of the thematic type.
Of the Thematic Verbs a large number remain. These may be divided into two classes: -
1. Verbs which preserve the thematic vowel e or o (in Latin i or u) before the personal endings. - These make up the Third Conjugation. The present stem is formed in various ways (§ 176), but always ends in a short vowel eo (Latin iu. Examples are tegó (stem tegeo-), sternimus (stem stern eo-) for ster-no-mos, plectunt (stem plect eo-) for plec-to-nti. So nóscó (stem gnósceo) for gnó-sc-ó. Verbs like nóscó became the type for a large number of verbs in -scó, called inceptives (§ 263. 1).
2. Verbs which form the present stem by means of the suffix yeo-, which already contained the thematic vowel eo. - Verbs of this class in which any vowel (except u) came in contact with the suffix yeo- suffered contraction so as to present a long vowel á-, é-, í-, at the end of the stem. In this contraction the thematic eo disappeared. These became the types of the First, Second, and Fourth conjugations respectively. In imitation of these long vowel-stems numerous verbs were formed by the Romans themselves (after the mode of formation had been entirely forgotten) from noun- and adjective-stems. This came to be the regular way of forming new verbs, just as in English the borrowed suffix -ize can be added to nouns and adjectives to make verbs: as, macadamize, modernize.
Thematic verbs of the second class in which a consonant or u came into contact with the suffix yeo suffered various phonetic changes. Such verbs fall partly into the Third Conjugation, giving rise to an irregular form of it, and partly into the Fourth, and some have forms of both. Examples are: - (cón) spició (-spicere) for spekyó; venió (veníre) for (g) vem-yó; cupió, cupere, but cupíví; orior, oritur, but orírí. Note, however, pluó (pluere) for plu-yó; and hence, by analogy, acuó (acuere) for acu-yó.
In all these cases many cross-analogies and errors as well as phonetic changes have been at work to produce irregularities. Hence has arisen the traditional system which is practically represented in §§ 175, 176.