([2.]) Metrical lengthening. On the lengthening of a vowel (or a syllable) under the influence of verse-ictus, see [2505].
[124]. A vowel originally long is regularly shortened in classical Latin before another vowel, even though an h intervene: as,
taceō, I am silent, from the stem tacē- ([365]); seorsum, apart, deorsum, downward, from sē(v)orsum, dē(v)orsum ([153]).
[125]. In simple words a diphthong occurs before a vowel only in one or two proper names: as, Gnaeus, Annaeus, in which it remains long, and in Greek words. But the diphthong ae of the prefix prae is sometimes shortened before a vowel: as, pra͝eacūtus; pra͝eeunt; pra͝ehibeō; hence prehendō for *prae-hendō. Sometimes it coalesces with a following vowel: as, pra͡e͡optāvīstī.
[126]. An increased tendency to shorten a long vowel before another vowel can be traced in the history of the language: thus, classical fuī, I was, for Plautus’s fūī ([750]); clueō, I am called, for Plautus’s clūeō; perfect pluit, it rained, for Varro’s plūit (cf. plūvit, [823], [947]); pius, pious, for Ennius’s pīus; see also [765].
[127]. But even in classical Latin there are cases where a vowel before another vowel remains long: thus,
([1.]) Regularly, the ī of fīō, I am made, except before -er-, as in fierem ([788], [789]).
([2.]) In dīus, godly, for dīvus ([153]), and the old ablatives dīū, dīō, open sky (used only in the expression sub dīū, sub dīō, i.e. sub dīvō).
([3.]) In the ending ēī of the genitive and dative sg. of stems in -ē- ([601]) when an i precedes: as, diēī, of a day, aciēī, of the battle line, but reī, of the thing, for older rēī.