DEBER vs DEBER DE

DEBER + INFINITIVE:

When the verb “deber” is followed by an infinitive verb, expresses obligation. It’s similar to “must, should or ought”.

No debes tomar esas pastillas. You must not take those pills.

¿Qué debo hacer? What should I do?

However, it’s also admitted to express probability by RAE.

¿Cuántos años tiene tu amiga? No estoy segura. Debe (de) tener 23 o 24.

How old is your friend? I´m not sure. She must be 23 or 24 years old.

When “deber” is used to express obligation, the conditional tense is usually used instead of the present tense when you want to sound more polite.

No deberías abrir la ventana si estás resfriado. You shouldn´t open the window if you are cold.

Deberías gastar menos dinero hasta que encuentres otro trabajo. You should spend less money until you find another job.

DEBER DE + INFINITIVE:

When the verb “deber” is followed by the preposition “DE” and an infinitive verb, expresses likelihood. It´s similar to “must” when “must” is not used for obligation.

¿Dónde está Luis? Son las 5 de la tarde, debe de estar en casa. Where is Luis? It´s 5pm, he must be at home.

¿Por qué no está aquí? No lo sé, debe de estar ocupado. Why is not he here? I don´t know, he must be busy.

*Note: it is possible you hear “deber de” to express obligation but remember: it is not correct. Don´t learn the same native speakers´mistakes