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.
The Essential AG: 204