What’s the difference between deber and deber de?

Introduction

The two verbal periphrasis deber + infinitive and deber de + infinitive have different meanings: deber + infinitive expresses an obligation and can be translated as must or be obliged to, while deber de + infinitive can only express probability or supposition, and can be translated as must be or must have.

Todos los conductores deben respetar el código de circulación y conducir con precaución.

El conductor de este vehículo debe de haber conducido muy deprisa.

deber + infinitivo

  • The verbal periphrasis deber + infinitive expresses an obligation.
  • It can be translated into English as must or be obliged to.
  • Grammatically, the verb deber acts as a modal verb, it is conjugated to show the tense and mood in which the action occurs. The infinitive that follows expresses the action of the sentence.
    Examples:
    Todos las opiniones deben ser respetadas.All opinions must be respected.
    El alquiler debe pagarse el día uno de cada mes.The tenant must pay on the first of the month.

deber de + infinitivo

  • The verbal periphrasis deber de + infinitive is used for deductions. It expresses probability, uncertainty and supposition.
  • It can be translated into English as must be or must have.
Example:
Carlota aún no ha llegado. Debe de haberse quedado dormida.Carlota hasn’t arrived yet. She must have overslept.

deber or tener que + infinitive?

Similar to the English must for obligation, deber is also typically limited to formal and written contexts. The more typical way to express obligation in Spanish is tener que + infinitive, which is comparable to the English have to.

Examples:
Debemos salir ya, o llegaremos tarde.We must leave now or we will be late.
Tenemos que salir ya, o llegaremos tarde.We have to leave now or we’ll be late.

However, deber and tener que are not always interchangeable. Sometimes deber is used as should; it expresses an obligation that is more subjective, gives advice or presents an opinion.

Example:
Debemos darnos prisa.We should hurry up.

In contrast, tener que expresses an obligation based on objective criteria.

Example:
Tengo que marcharme; tengo una cita.I have to go; I have an appointment.

For this reason, deber and tener que are not used indiscriminately in different tenses:

  • Deber is mostly used in the present tense and in the conditional. It is rarely used in the future and preterite tenses or in the subjunctive mood.
    Examples:
    Debes hablar con propiedad.You must speak properly.
    No debes/deberías decir palabrotas.You mustn’t/shouldn’t say swear words.
    As deber can be used to give advice on something the speaker believes is necessary, using it in the past tense has the opposite meaning:
    Example:
    Debiste ser más educado. → no fuiste educadoYou should have been more polite. → You were not polite.
  • On the other hand, tener que can be used in any tense or mood.
    Examples:
    Todos los pasajeros tuvieron que presentar su pasaporte en la aduana. (preterite)All the passengers had to show their passport at customs.
    Todos los pasajeros tienen que presentar su pasaporte en la aduana. (present)All the passengers have to show their passport at customs.
    Todos los pasajeros tendrán que presentar su pasaporte en la aduana. (future)All the passengers will have to show their passport at customs.
    Es lógico que todos los pasajeros tuvieran que presentar su pasaporte en la aduana. (imperfect subjunctive)It’s logical that all the passengers have to show their passport at customs.