This is an irregular verb. It follows the conjugation pattern of the verb tener.
Stem Change: e ie (mantienes)
Present 1st Person: mantengo
Imperative 2nd Person: mantén
Preterite Stem: mantuv-
Preterite Endings: e - iste - o - imos - isteis - ieron
Future Stem: mantendr-
PresentPresente
Yo mantengo contacto con mis amigos de la escuela.
I keep in touch with my school friends.
PreteritePretérito indefinido
Ellos mantuvieron la calma durante la tormenta.
They kept calm during the storm.
FutureFuturo
El restaurante mantendrá sus precios este año.
The restaurant will keep its prices this year.