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.
- fierem, fierēs, fieret; fierī
The Essential AG: 603.3, 606.3a