What's the difference between deber and deber de?

Deber + infinitive = obligation / duty (should, ought to). Deber de + infinitive = probability / inference (must be, a guess). Debes estudiar (You should study) vs. Debe de ser tarde (It must be late).

Can I drop the de in deber de?

Native speakers often do, even when expressing probability. RAE officially preserves the distinction (deber for duty, deber de for guess). Both Debe ser tarde and Debe de ser tarde are accepted in modern Spanish.

How is deber different from tener que?

Deber implies moral duty or expected behavior (should, ought to). Tener que implies necessity (have to). Tienes que estudiar (you have to) is more imperative than Debes estudiar (you should).

How do I say should have in Spanish?

Conditional or imperfect of deber + haber + past participle. Debería haber estudiado (I should have studied), Debías haber llamado (You should have called). Common for past regrets.

How can I master deber in Spanish?

Practice deber + infinitive for duty (Debo estudiar), and deber de + infinitive for probability (Debe de ser tarde). Conditional for softer suggestions (Deberías). Native input through Parrot videos models each.

Spanish grammar · Intermediate

Deber and Deber de + Infinitive: Obligation and Probability

Written by Sofia Ramirez, Lead Spanish Curriculum Editor · Reviewed May 2026

Deber + infinitive expresses obligation or duty (should / ought to). Deber de + infinitive expresses probability (must be, inference). RAE technically distinguishes the two, but native speakers often use them interchangeably.

Debes estudiar más.

You should study more.

What it is

Spanish has TWO closely related constructions with deber. Deber + infinitive (no de) expresses OBLIGATION or DUTY, should, ought to (Debes estudiar = You should study). Deber DE + infinitive expresses PROBABILITY or inference, must be (Debe de ser tarde = It must be late). RAE officially distinguishes them; in actual speech, the de is often dropped or added interchangeably.

Debes estudiar más (You should study more), duty. Debe de tener veinte años (He must be around twenty), probability / guess.

How to spot it

Look for deber conjugated + infinitive. Check context: if it's about what someone SHOULD do, no de. If it's a guess / inference, de is preferred (but optional in practice).

Debes estudiar más. — You should study more. (obligation)
Debe de ser tarde. — It must be late. (probability)
Deberíamos llegar a tiempo. — We should arrive on time.

Spanish speakers often drop the de even when expressing probability. Both Debe ser él and Debe de ser él are widely accepted in modern Spanish.

Deber (de) + Infinitive Quick Reference

Deber vs. deber de

Construction	Meaning	Example
deber + inf	should / ought to (duty)	Debes estudiar.
deber de + inf	must be (probability / guess)	Debe de ser tarde.
deber conditional	softer should	Deberías estudiar.
deber imperfect	past duty	Debía estudiar.

Common Deber (de) + Infinitive Examples in Spanish

Deber in real Spanish contexts:

Obligation / Duty

Debes estudiar más.: You should study more.
Debemos respetar las reglas.: We must respect the rules.
Deben llegar a tiempo.: They have to arrive on time.

Deber + infinitive for duty / moral obligation. Slightly softer than tener que.

Probability / Inference (Deber De)

Debe de ser tarde.: It must be late.
Deben de estar en casa.: They must be at home.
Debías de estar cansado.: You must have been tired.

Deber de + infinitive for guesses, inferences, what's probably true.

Softer Suggestions (Conditional)

Deberías estudiar más.: You should study more. (softer)
Deberíamos llegar pronto.: We should arrive soon.

Conditional of deber softens the obligation. Less direct than present indicative.

Past Obligation

Debías haber llamado.: You should have called.
Debería haber estudiado.: I should have studied.

Conditional / imperfect of deber + haber + participle = should have / should have done.

How to Use Deber and Deber de + Infinitive

Deber + Infinitive = Should / Must (Duty)

Conjugate deber and add an infinitive. Expresses obligation or moral duty, should, ought to.

Debes estudiar. Debemos ayudar. Deben llegar a tiempo.

You should study. We must help. They have to arrive on time.

Slightly softer / more moralistic than tener que. Deber implies a sense of duty, not just necessity.

Deber De + Infinitive = Probably (Inference)

Add de between deber and the infinitive to express probability / inference, must be (a guess).

Debe de ser tarde. Deben de estar cansados.

It must be late. They must be tired.

RAE officially distinguishes deber (duty) and deber de (probability), but native speech often blurs them. Use de when expressing inference for clarity.

Conditional Softens the Obligation

Use conditional (debería, deberías, deberíamos) for a softer should, suggestions, advice.

Deberías estudiar más. Deberíamos llegar a tiempo.

You should study more. We should arrive on time.

Conditional = polite suggestion. Present indicative = stronger obligation.

Past Form: Should Have

Conditional / imperfect of deber + haber + past participle = should have / ought to have.

Debería haber estudiado. Debías haber llamado.

I should have studied. You should have called.

Common for past regrets or missed obligations. Imperfect debía haber = should have (slightly more direct than conditional).

Common Mistakes with Deber (de) + Infinitive

Incorrect: Debe que ser tarde. — It must be late.

Correct: Debe de ser tarde. (or Debe ser tarde.) — It must be late.

Don't confuse with tener que. Deber takes infinitive directly (no que). For probability, add de (optional in modern usage).

Incorrect: Debo de estudiar. — I should study. (probably wrong with de)

Correct: Debo estudiar. — I should study.

For DUTY (should / ought to), no de. Deber de is for probability / inference, not obligation.

Incorrect: Debes a llamar. — You should call.

Correct: Debes llamar. — You should call.

Deber takes infinitive directly, no a, no que. Just deber + infinitive.

Deber vs. Deber De, Obligation vs. Probability

Deber + Inf = Should / Ought To (Duty)

Use without de for obligation, moral duty, or expectation.

Debes ayudar a tu hermano.: You should help your brother.
Debemos respetar las reglas.: We must respect the rules.

Slightly softer than tener que. Conveys a sense of moral / expected duty.

Deber De + Inf = Must Be (Probability)

Use with de for inferences, guesses, or what's probably true.

Debe de ser tarde.: It must be late. (it's probably late)
Deben de tener hambre.: They must be hungry.

Modern speech often drops the de. RAE officially keeps the distinction. Both Debe ser tarde and Debe de ser tarde are widely heard.

Deber (de) + Infinitive FAQs

What's the difference between deber and deber de?: Deber + infinitive = obligation / duty (should, ought to). Deber de + infinitive = probability / inference (must be, a guess). Debes estudiar (You should study) vs. Debe de ser tarde (It must be late).
Can I drop the de in deber de?: Native speakers often do, even when expressing probability. RAE officially preserves the distinction (deber for duty, deber de for guess). Both Debe ser tarde and Debe de ser tarde are accepted in modern Spanish.
How is deber different from tener que?: Deber implies moral duty or expected behavior (should, ought to). Tener que implies necessity (have to). Tienes que estudiar (you have to) is more imperative than Debes estudiar (you should).
How do I say should have in Spanish?: Conditional or imperfect of deber + haber + past participle. Debería haber estudiado (I should have studied), Debías haber llamado (You should have called). Common for past regrets.
How can I master deber in Spanish?: Practice deber + infinitive for duty (Debo estudiar), and deber de + infinitive for probability (Debe de ser tarde). Conditional for softer suggestions (Deberías). Native input through Parrot videos models each.