Deber + Infinitive (Should/Must) | Spanish (Latin America) Grammar

The verb deber + infinitive expresses moral obligation, duty, or a strong recommendation. It translates to "should", "must", or "ought to" in English. Unlike tener que, which conveys a concrete, practical necessity, deber carries a sense of moral correctness or social expectation.

There's also a related construction, deber de + infinitive, which expresses probability rather than obligation.

Formation (Obligation)

Conjugate deber in any tense and add an infinitive directly—no preposition.

Subject	Deber (present)	Infinitive
yo	debo	debo estudiar
tú	debes	debes estudiar
él/ella/usted	debe	debe estudiar
nosotros	debemos	debemos estudiar
ustedes/ellos	deben	deben estudiar

Debes estudiar más si quieres aprobar.

You should study more if you want to pass.

Debemos respetar a nuestros mayores.

We must respect our elders.

Conditional to Soften

The conditional form debería + infinitive softens the obligation into friendly advice or a polite recommendation. It's extremely common in everyday Latin American Spanish.

Form	Tone	Example
debes	Direct, strong	Debes ir al médico.
deberías	Softer, advisory	Deberías ir al médico.

Deberías descansar un poco más.

You should rest a bit more.

Deberíamos llamarla para ver cómo está.

We should call her to see how she's doing.

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Use deberías when giving friendly advice. It's the Spanish equivalent of English "you should..." in a gentle, non-imposing way. Debes sounds more like "you must..." or "you need to..."—firmer and more direct.

Deber de + Infinitive: Probability

When you add the preposition de, the meaning changes completely. Deber de + infinitive expresses probability or conjecture—what you guess must be true.

Deben de ser las nueve de la noche.

It must be about nine at night.

Debe de estar cansado después de trabajar tanto.

He must be tired after working so much.

Deber vs. Deber de

The two structures are often confused, even by native speakers:

Structure	Meaning	Example
deber + infinitive	obligation, duty	Debes estudiar.
deber de + infinitive	probability, guess	Debes de estar cansado.

Debes estar en casa a las diez.

You must be home at ten (it's an obligation).

Debes de estar en casa porque nadie responde al celular.

You must be at home since no one is answering the phone.

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In practice, many Latin American speakers drop the de even when expressing probability, so you'll often hear debe ser tarde ("it must be late") used to mean the same as debe de ser tarde. The distinction is clearer in formal writing.

Past Uses

Debí + infinitive (preterite) and debería haber + past participle (conditional perfect) express past regret or criticism—"I should have...".

Debí haberte avisado antes, lo siento.

I should have told you sooner, I'm sorry.

Deber vs. Tener Que vs. Hay Que

All three express obligation, but each has a different flavor:

tener que — personal, concrete, unavoidable
deber — moral, advisory, ethical
hay que — impersonal, general, abstract

See tener que + infinitive and hay que + infinitive for a fuller comparison.