Échoir is a defective verb. It can not be conjugated in all possible forms.
Here are several examples of defectives verbs:
paître.
Échoir verb is conjugated in female form and with auxiliary être. Be careful, the past participle of compound tenses must agree with the subject.
Échoir 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 »:
« échoira-t-elle ">« sera-t-elle échue ? ».
Finally, the interrogative form does not exist in subjunctive and imperative.