Tréfondre 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
-oudre).
Prendre and its compounds, however, have a very different conjugation, where the
d is only kept in the future and conditional.