Vaincre is a french third group verb. So, this verb is irregular and do not follow a regular conjugation pattern like first or second group verbs. Follow this link to see all the endings of the conjugation of most of the third group verbs :
conjugation rules and endings for the second group verbs.
Vaincre verb is conjugated in interrogative form. For sound reasons, an euphonious
« t » is added to the pronoun of the third person:
« il »,
« elle »,
« on »,
« ils »,
« elles » except when the verb ends with
« t » or
« d »:
« vainc-t-il ">« aura-t-il vaincu ? ».
Finally, the interrogative form does not exist in subjunctive and imperative.