¿Cuál es el origen de la terminación en -o para el pretérito de tercera persona singular?

Question

En las tres conjugaciones de verbos en español, la tercera persona singular forma su pretérito simple con terminación en -o, como «amó», «tuvo» o «dijo». Pero la forma de la que provienen en latín, a saber el perfecto, tenía terminación -avit, -ait, -uit, -it; en general -it final.

Cuando evolucionó hacia la forma española moderna, la forma para la primera persona singular que solo difería por la -t final de aquella de la tercera, resultó en una terminación con -e como «amé», «tuve» o «dije» y siguiendo el paradigma de otras palabras similares, la 3.p.s. debió haber resultado igual pues la pronta pérdida de la -t final habría dejado las formas de ambas personas iguales. Sin embargo hoy en día tenemos que este tiempo termina en -o, ¿por qué? ¿Qué origen tiene?

Answer 1

Some digging around on the internet reveals that Ian Mackenzie at the University of Newcastle (UK) knows the answer. The relevant page on verbs is here. In that page there are some links to other material which have not copied across in my version below. I know your question is in Spanish but I hope the English version below helps you.

In contrast to what happened in the first person singular, the consonantal element -v- appears to have been retained in the third person singular, and the contraction consisted in the elision of the vowel ĭ that came immediately after it. The endings -āvĭt and -īvĭt were thus reduced to [-awt] and [-iwt] respectively.

In the -āre ending, [aw] underwent its normal reduction to /o/, producing the ending -ot which is found in early Old Spanish. In the -īre ending, the semivowel [w] became syllabic, i.e. [ˈiwt] > [ˈiut]. As happened generally, /u/ in final syllables was replaced by /o/, resulting in the early Old Spanish ending -iot, initially pronounced [ˈiot] but over time evolving to [ˈjot]. The final /t/ of both -ot and -iot went the way of Latin final /t/ generally, not surviving much beyond the eleventh century. Following this latter development, the third person singular endings were as they are in modern Spanish, namely -ó and -ió (although the practice of indicating the stress with a written accent dates from the early modern period).