Tener que vs. deber

Similar to 'must' and 'to have to' in English, deber and tener que are both used to express, on the one hand, obligation and necessity, and on the other hand, speculation or probability.

Obligation/Necessity (modal radical)

Similar to 'must', deber is used to express an internal or moral obligation, usually imposed by the speaker or social norms; it is used far less in everyday spoken Spanish.

"La obligación expresada por «deber + infinitivo» está en consonancia con lo generalmente deseable o con normas aceptadas que así lo imponen".  RAE

Like 'to have to', tener que is used for a strong external obligation, imposed by the circumstances.

"[...] expresa obligación o necesidad externa inevitable impuesta por las circunstancias, de forma que el sujeto no es capaz de sustraerse a ella". RAE

In many cases they can be used interchangeably, with tener que being far more frequent in spoken Spanish. However, in the following example the use of deber would sound less natural since the thing needing done goes against social norms and what would be considered to be a person's 'deber'.²

-Las autoridades tienen a veces que cometer actos ilegales — The authorities sometimes have to commit illegal acts

As discussed previously, the negations no tener que and no deber are used differently, although their meaning is sometimes the same.

 In the past

An important difference is when used in the past pretérito indefinido, tener que implies something that had to be done and was done, while deber implies something that needed/had to be done, but often it wasn't done.^1-2

-Tuvo que salir porque se sentía mareado — He had to go out because he felt dizzy [He went out]

→Debió salir porque el Juez pidió que se abandonara la sala — He had to go out because the judge asked everyone to leave the courtroom [Without more context we assume he did indeed go out]

→→Debió salir, pero no lo hizo — He was supposed to leave/should have left the room, but he didn't [≈Debió haber salido - clearly counterfactual without more context]

 

-Tuvieron que meterlo en la cárcel — They had to lock him up [They did]

→Debieron meterlo en la cárcel — They should have locked him up [≈Deberían/ debieron/ debían/ debieran haberlo metido en la carcel - se neutralizan semánticamente todas estas formas y la diferencia es mínima]

In the past imperfect they can both be used to talk about things that were needing done in the past, without telling us whether the thing was done or not in the end. [See imperfecto prospectivo]

-Trabajé rápido porque tenía que/debía marcharme esa tarde — I worked quicly because I had to/was supposed to leave that afternoon

 In the conditional, both tener que and deber can be used to talk about something that should be done. However, bear in mind that tener que may also mean 'would have to' when used in a conditional clause.

-Yo tendría que/debería estar en el trabajo ya — I should be at work already

-Debería haberlo/Tendría que haberlo hecho antes — I should have done it before [See previous article]

 

-Si quisiera ser bombero, tendría que ponerme en forma — If I wanted to be a fireman, I would have to get fit

In subordinate clauses, or when used with other perífrasis verbales, only tener que is possible - similar to English.

-No me gusta tener que (deber??) decírselo  — I don't like having to tell him

-Voy a tener que (deber??) hacerlo  — I'm going to have to do it

Speculation/Probability (modal epistémico)

Both tener que and deber can be used for deduction and to express probability, with tener que being the stronger of the two, much like 'have (got) to' in English.

Deber, when used for deduction, is preferred with the preposition de, i.e. deber de, although it is often seen without. Use with de for obligation is recommended against by the RAE.³

-Debe (de) estar en su casa — He must be at his house [preferred with 'de' for speculation]

→Debes quedarte en casa — You must stay at home ['de' shouldn't be used for obligation]

-Tiene que estar en su casa — He has (got) to be at his house [stronger]

 

-Debe (de) haber una solución — There must be a solution [preferred with 'de' for speculation]

-Tiene que haber una solución — There's got to be a solution [more emphatic]

In the past

Similarly, both can be used for speculation in the past.

As discussed previously, since the modal verbs in Spanish can be used in many tenses, the various translations can be confusing for English speakers.

Consider the following example which, seen in isolation, is far more ambiguous in English than in Spanish.

• For difference between necesitar and tener que see discussion.

In Español-Avanzado Articles

-Sherlock Holmes - Radio Teatro

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-tener que vs deber + infinitivo-