Learn Latin from the Romans A Complete Introductory Course Using Textbooks from the Roman Empire

The verb fīō is in effect the passive of faciō in the present, future, and imperfect tenses (i.e. those tenses formed from the first two principal parts); its principal parts are therefore fīō, fierī, factus sum. Strictly speaking fīō is not very irregular, for nearly all its forms can be formed regularly from the principal parts if one classes it as a mixed conjugation partially deponent verb. Nevertheless this formation process is tricky enough that fīō is normally classed with the irregular verbs.