This is an irregular verb. It follows the conjugation pattern of the verb poner.
Present 1st Person: supongo
Imperative 2nd Person: supón
Preterite Stem: supus-
Preterite Endings: e - iste - o - imos - isteis - ieron
Future Stem: supondr-
Participle: supuesto
PresentPresente
Yo supongo que el tren llega a las cinco.
I suppose the train arrives at five.
PreteritePretérito indefinido
Todos supusimos que la reunión era hoy.
We all supposed the meeting was today.
ConditionalCondicional
Yo supondría lo mismo en tu lugar.
I would suppose the same in your place.
Imperfect SubjunctiveImperfecto de subjuntivo
Si supusiéramos lo peor, nunca saldríamos de casa.
If we assumed the worst, we would never leave the house.