This is an irregular verb. It follows the conjugation pattern of the verb poner.
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.