As I discussed in the last post, certain forms of fīō feature a short i instead of the usual long. I thought there would be a long and complicated phonological history to tell, but it turns out fīō is just following a few phonological rules that we already know.
Long vowels before final m, r and t are shortened.
amō, amās, amat
amem, amēs, amet
amer, amēris, amētur
so also fīō, fīs, fit
The second rule is more exclusive to fīō, but follows a consistent pattern: the ī is shortened before -er.
Compounds of faciō vary between passives in -fīō and passives in -ficior. The distinction? Check the vowel a (faciō) in the compound. In the rare case that this is retain in the compound, then –fīō is also retained.
benefaciō, benefacere, benefēcī, benefactum (in place of the expected beneficio/ficere/fēcī/fectum, and hence the English ‘benefaction’ but also ‘infection.’)
benefīō, benefierī, benefactus sum
Several of the faciō compounds that feature -ficiō/-ficior forms will also feature passive -fīō forms, with separate meanings.
In place of a passive form [facior], Latin makes use of fīō, fierī, factus sum.
Note that both the ī and the ō are long, which distinguishes fīō from similar –iō verbs, where the i is short
Note that A&G get very prescriptive about the proper forms of fīō, and whereas they distinguish between those forms which appear in “good” prose from those which appear in (“bad”?) prose, we will make no such distinction here.
photo credit: Wiktionary
Note a few variations: the long vowel on ī is present in present in most places, but absent in fit, fierem, and fierī.