There are 13+ verbs that do not conform to the regular formation of the verbs in the conditionnel (infinitive of the verb + inparfait endings); They are avoir, aller. etre, faire, devoir, pouvoir, recevoir, pleuvoir, voir,   falloir, savoir. venir, vouloir. THESE change their stem to>>> aur-, ir-, ser-, fer-, devr-, pourr-, recevr-, pleuvr-, verr-, faudr-, saur-, viendr-, and voudr-.

There are other verbs who have slight changes the infinitive such as Yer- verbs>. the Y turns into an “I’. With Acheter, the accent grave is added in the L formation.