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.
- cōnfit, it happens
- dēfit, it lacks
- īnfit, he beings (to speak)
- interfit, he perishes
- superfit, there remains
The Essential AG: 204b-c