The conditional may be safely used to translate ‘ought’:
Deberías vestirte de etiqueta. You ought to dress formally.
In formal Spanish in the Peninsula but more generally in Latin America,
the -raform of the imperfect subjunctive can be used in the same way:
Debieran confesar su error. They ought to own up to
their mistake.
These tenses may also be used to refer to the past when followed by a
perfect infinitive:
Deberíamos/Debiéramos haberle negado el permiso de
residencia.
We ought to have refused him his residence permit.
Alternatively to refer to the past, the choice of the preterite or imperfect of
deberwith the meaning of ‘must have’, ‘should have’ or ‘ought to have’,
is based on the tense that would have been used if deberhad not been
needed:
Debía(de) tener tres hijos. He must have had three
children.
(based on teníatres hijos) (based on ‘he had three
children’)
Debíeligir la otra opción. I should have chosen the
other option.
(based on eligíla otra opción) (based on ‘I chose the other
option’)
Debía(de) ser medianoche It must have been midnight
cuando volvieron. when they returned.
(based on eramedianoche) (based on ‘it was midnight’)Tener que
Interchangeable in most contexts with deber, tener questates a strong
obligation viewed as a necessity, comparable to English ‘have to’. When
used in the preterite tense, the meaning is ‘had to’ and did:
Tendrás que cambiar de You will have to change planes
avión en París. in Paris.
Tuvo que presentar la tesis He had to present the thesis
de nuevo. again.16.2
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Tener que