1. As we have seen, thematic stems in -s- are often Desideratives in the Present (and they sometimes function as immediate Futures); with secondary endings, they can form Imperfects.
NOTE. Forms in -so are often found in Slavic; as, vedu / veso, reco / reso, etc.
The -s- is, so, added to a Root ending:
a. In consonant with long vowel, in contrast with the Present with full vowel.
b. In -a, -e, -o with the same stem as the Present or the noun from which the verb is derived. There is also a second Imperfect mark: an -e before the -s-, which is probably an older Imperfect, to which another Imperfect mark is added; as in alko / alkeso, grow; mnio / mnso, be mad; etc.; -e and -a must have Ø root grade in the Imperfect stem.
NOTE. The thematic Imperfects are mostly used as Presents in the Southern Dialects, in Slavic, and, probably, in Latin.
2. The most spread athematic stems are in -s-. They were formerly added to the Root, wether monosyllabic or disyllabic, in consonant or vowel, opposed thus to the Present. Roots in -i, -u, -a, -e, -o, monosyllabic and disyllabic, have a fixed vowel (as most of the athematic Root Imperfects); as, the third person pl. plaent from pipel, full (Ø / Full Grade), 3.pl. pewisent from pona, purifie (Full Grade / o).
The most frequent are monosyllabic roots ending in consonant or sonant. They have usually in the Southern Dialects lengthened vowel in the active voice and Ø in the rest; as, liq-, leave, from which leiq-s-m; so from qer-, make, qerso; from bher-, carry, bherso, etc.
To sum up, the -s- has one main function in Europaio: to create stems different from that of the Present. It can be added to different stems, especially those lengthened with -e, -a.
NOTE. The Imperfect in -s- is then a modern feature of IE III, given in all its dialects (with the form of Aorists in the Southern ones), but for Germanic and Baltic; the derivatives in -i, -e, -a are more recent, though, and therefore not extended to all.
3. There are some other frequent, dialectal forms:
a. -is (lat., ai.), -es (gr.); as, genist from gen-, beget; wersis from wers-, rain; amawis, etc.
b. An addition in -sa in lat., toc., arm.
c. In -se, -sie/-sio, etc.
4. The stems in -t function usually as Preterites, especially in lat., ita., cel., and ger., opposed to the Present. In our system, they appear usually in the fourth declension.
NOTE. While the use of -t for persons is old, the use to form Stems is more recent, and mainly developed by the European dialects.
5. Imperfects or Pasts in -k are rare, but they can appear in every form of the verb.