deber vs deber de
must (duty) vs must (guess)
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must (duty) vs must (guess)
Deber + infinitive commonly expresses obligation; deber de + infinitive commonly expresses a supposition. Deber without de can also express a supposition, and actual usage overlaps.
Examples in context
Read the examples and compare their English meanings.
Obligation / Duty
No preposition means a clear rule or obligation.
Debes entregar el informe hoy.
You must submit the report today.
Probability / Guess
Adding 'de' signals a strong assumption or guess.
Debe de costar una fortuna.
It must cost a fortune (I bet).
Common mix-ups
Deber + infinitive is the usual choice for an obligation. Deber de often signals a supposition, though actual usage overlaps.
You must arrive at the meeting on time.
Try a reply
Say or write your answer in Spanish before opening the example.
You guess that Ana is about forty. Use deber de.
Show one possible answer
Ana debe de tener unos cuarenta años.
Other wording can work too. Compare the verb form, agreement, and word order with the lesson. Then try a sentence about your own day.
Keep practicing
Related grammar
Grammar references
Practice Spanish grammar in context
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