OFr verbs fall into three classes, and can be grouped according to their infinitive endings, namely:
Class Ia: Infinitives in -er, e.g. chanter.
Most verbs belong to this class (well over 1000 in the twelfth century).
Class Ib: Infinitives in -ier e.g. laissier.
These are fairly numerous (well over 500 in the twelfth century). They are conjugated like Class ia verbs, except for a diphthong ie instead of e in the infinitive and in five other verb forms.
Class II: Infinitives in -ir, present participle with the infix -iss-, e.g.
This class contains a few score verbs in the twelfth century, but the number slowly increases with the absorption of -ir verbs from Class III.
Class III: Infinitives in -ir, -re, and -eir > oir, e.g. servir, dire, deveir > devoir.
Nearly 200 verbs belong to this class in the twelfth century, and many show irregular forms. Some of the -ir verbs however gradually adopt the infix -iss- and are absorbed into Class II (§68.2).
Tense endings of all verb classes are given in Table 5. A sample verb from each class is conjugated in Table 6 and the verb boivre has been added as an example of vocalic alternation (§§65, 76). For the auxiliary verbs avoir (to have) and estre (to be) see Tables 7 and 8.
Early twelfth-century endings which provide more information are shown in Table 5, but standard twelfth-century forms (after 1150) are used in Table 6 for comparison.