Retondre 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
Most verbs in -dre
: -andre, -endre, -ondre, -erdre, -ordre
conjugate like rendre
(major exceptions are those in -indre
and its compounds, however, have a very different conjugation, where the d
is only kept in the future and conditional.