In the history of the vowels and diphthongs in Latin we must distinguish the changes which came about independently of accent and those produced by the preponderance of accent in another syllable.
17. Vowel Changes independent of Accent.—In the former category the following are those of chief importance:—
(i.) ĭ became ĕ (a) when final, as in ant-e beside Gr. ἀντί, trīste besides trīsti-s, contrasted with e.g., the Greek neuter ἴδρι (the final -e of the infinitive—regere, &c.—is the -ĭ of the locative, just as in the so-called ablatives genere, &c.); (b) before -r- which has arisen from -s-, as in cineris beside cinis, cinisculus; serō beside Gr. ἴ(σ)ημι (Ind.-Eur. *si-sēmi, a reduplicated non-thematic present).
(ii.) Final ŏ became ĕ; imperative sequere = Gr. ἔπε(σ)ο; Lat. ille may contain the old pronoun *so, “he,” Gr. ὁ, Sans. sa (otherwise Skutsch, Glotta, i. Hefte 2-3).
(iii.) el became ol when followed by any sound save e, i or l, as in volō, volt beside velle; colō beside Gr. τέλλομαι, πολεῖν, Att. τέλος; colōnus for *quelōnus, beside inquilīnus for *en-quēlenus.
(iv.) e became i (i.) before a nasal followed by a palatal or velar consonant (tingo, Gr. τέγγω; in-cipio from *en-capio); (ii.) under certain conditions not yet precisely defined, one of which was i in a following syllable (nihil, nisi, initium). From these forms in- spread and banished en-, the earlier form.
(v.) The “neutral vowel” (“schwa Indo-Germanicum”) which arose in pro-ethnic Indo-European from the reduction of long ā, ē or ō in unaccented syllables (as in the -tós participles of such roots as stā-, dhē-, dō-, *stƏtós, *dhƏtós, *dƏtós) became a in Latin (status con-ditus [from *con-dhatos], datus), and it is the same sound which is represented by a in most of the forms of dō (damus, dabō, &c.).
(vi.) When a long vowel came to stand before another vowel in the same word through loss of ḭ or ṷ, it was always shortened; thus the -eō of intransitive verbs like candeō, caleō is for -ēḭō (where the ē is identical with the η in Gr. ἐφάνην, ἐμάνμν) and was thus confused with the causative -eiō (as in moneō, “I make to think,” &c.), where the short e is original. So audīuī became audīī and thence audiī (the form audīvī would have disappeared altogether but for being restored from audīveram, &c.; conversely audieram is formed from audiī). In certain cases the vowels contracted, as in trēs, partēs, &c. with -ēs from eḭes, *amō from amā(ḭ)ō.
18. Of the Diphthongs.
(vii.) eu became ou in pro-ethnic Italic, Lat. novus: Gr. νέος, Lat. novem, Umb. nuviper (i.e. noviper), “usque ad Changes of the diphthongs independent of accent. noviens”: Gr. (ἐν-)νέα; in unaccented syllables this -ov- sank to -u(v)- as in dếnuō from dế novō, suus (which is rarely anything but an enclitic word), Old Lat. sovos: Gr. ἑ(ϝ)ός.