Deber preterite conjugation

Deber means must, should, or ought.
Deber is regular in the preterite.

Sentences using deber in the preterite tense

Debí pensarlo dos veces.
      I should have thought about it twice.

Tú debiste estudiar todos los libros.
      You should have studied all the books.

Él debió hacerlo.
      He should have done it.

Usted debió haber llamado a la policía
      immediatamente.
     You should have called the police
      immediately. (formal)

Debimos contener el fuego.
      We should have contained the fire.

No debieron olvidar sus derechos.
      They ought not to have forgotten their rights.