Deber versus Deber de (obligation and assumption)

The verb deber (must) in Spanish has a specific use and meaning when it is followed by preposition "de". We use it to express a strong assumption about the action in the main verb used in the sentence.

Have a look and listen to the following examples:

Ese bolso debe de costar una fortuna.
That handbag must cost a fortune. (it very likely cost a fortune)

Las chicas deben de llegar pronto.
The girls must be arriving soon. (it's very likely they will arrive soon)

Ese cuadro debe de ser de Matisse.
That painting must be a Matisse. (it's very likely a Matisse)

As a general rule, deber directly followed by an infinitive (without de) means obligation although its use to express assumption is also accepted:

Margarita debe de tener unos 37 años.
Margarita is probably about 37 years old.

Margarita debe tener unos 37 años.
Margarita is probably about 37 years old.

Both sentences above are correct, expressing a strong assumption.

Let's see another example:

Debe haber un cartel anunciando el horario.
There must be a sign showing the opening hours. (obligation) 
Surely there is a sign showing the opening hours.  (strong assumption)

Debe de haber un cartel anunciando el horario.  
Surely there is a sign showing the opening hours.  (strong assumption)

You can see above that deber + infinitive could mean both, but deber de + infinitive is only considered an assumption, not an obligation.

Bear in mind that deber (de) can also be used in other tenses:

For example:

Margarita debe de tener unos 37 años.
Margarita is probably about 37 years old.

Margarita debe tener unos 37 años.
Margarita is probably about 37 years old.

And can also be followed by El Infinitivo Compuesto when we are talking about assumptions related to past actions.

For example:

Eso debe de haber caducado hace tiempo.
That must have expired some time ago

No encuentro mi móvil. Debo de haberlo dejado en el coche.
I can't find my mobile phone. I have probably left it in the car.

 

Be careful with verbs that have a pronoun like the last example (lo). You can attach it to "haber" or place it right in front of the whole verbal structure, but not after the participle.

Lo debo de haber dejado en el coche.
Debo de haber dejadolo en el coche."

To learn about other ways to express probability/assumption see: 

Learn more about these related Spanish grammar topics

Examples and resources

El tren debió de llegar sobre las cuatro y media.
The train must have arrived at about four thirty.


No encuentro mi móvil. Debo de haberlo dejado en el coche.
I can't find my mobile phone. I have probably left it in the car.


Margarita debe de tener unos 37 años.
Margarita is probably about 37 years old.


Ese cuadro debe de ser de Matisse.
That painting must be a Matisse. (it's very likely a Matisse)


Eso debe de haber caducado hace tiempo.
That must have expired some time ago


Las chicas deben de llegar pronto.
The girls must be arriving soon. (it's very likely they will arrive soon)


Margarita debe tener unos 37 años.
Margarita is probably about 37 years old.


Los chicos han debido de beber bastante porque huelen a alcohol.
They boys must have drunk quite a lot as they smell of alcohol.


Ese bolso debe de costar una fortuna.
That handbag must cost a fortune. (it very likely cost a fortune)


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