Défaillir 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.
Défaillir 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 »:
« défaille-t-elle ">« aura-t-elle défailli ? ».
Always in interrogative form, and always for sound reasons, an accent appears in first person present indicative tense when the verb ends with
« e ». An euphonious
« e » is replaced by
« é »:
« défaillé-je ? ».
Finally, the interrogative form does not exist in subjunctive and imperative.