What special term do we use for verbs like OFFERO,OFFERRE tht don't fit into a specific conjugation?.
Verbs like OFFERO, OFFERRE that don't fit into a specific conjugation are called Irregular Verbs for they have an irregular conjugation that you must learn by heart.
Such Irregular Verbs are SUM,VOLO,FERO,EO,QUEO,FIO, and their compounds.
For example, OFFERO is a compound of FERO (OF-FERO)and then you must know the conjugation of FERO so that you can conjugate OFFERO.
So, the present indicative of FERO is “Fero, fers, fert, ferimus, fertis, ferunt” and then the present indicative of OFFERO is “Offero, offers, offert, offerimus, offertis, offerunt”.
As you can see, the conjugation of OFFERO follows FERO and its forms that you can read at Allen and Greenough's New Latin Grammar for Schools and Colleges:
To conclude, the Irregular Verbs must be learnt by heart, as they do not belong to the Four Regular Conjugations.
Hope this can be helpful to you. Feel free however to ask me again.