Different uses of deber in Spanish
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)
As a general rule, deber directly followed by an infinitive (without de) means obligation although its use to express assumption is also accepted:
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:
And can also be followed by El Infinitivo Compuesto when we are talking about assumptions related to past actions.
For example:
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:
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