Substituer is a french first group verb. So it follow the regular conjugation pattern of the first group like:
aimer.
Follow this link to see all the endings of the conjugation of the first group verbs :
conjugation rules and endings for the first group verbs.
Substituer is conjugated the same way that verbs that end in :
-er.
First group verbs always end with
-er as their infinitive. Endings of first group verbs are regular.
Note : Although endings are perfectly regular, spelling changes occur in the stems of verbs in
-cer,
-ger,
-yer,
-eter,
-eler,
-é-consonne(s)-er,
-e-consonne-er, verb
envoyer and similar verbs.
Substituer 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 »:
« substitue-t-il ? »,
« substitua-t-il ? »,
« substituera-t-il ? »,
« a-t-il substitué ? »,
« aura-t-il substitué ? ».
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 »:
« substitué-je ? ».
Finally, the interrogative form does not exist in subjunctive and imperative.
