Décommettre 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.
For all purpose but spelling,
battre and its derived verbs are conjugated like
rendre. The only difference is in spelling, as native French words may not have
tt before a consonant, thus the indicative present singular of these verbs goes
bats, bats, bat.
Mettre and its family show the same changes, but additionally have a past participle, past historic, and subjunctive imperfect in
mi-.