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Chapter 20: Modal verbs (Auxiliaries)

  20.6 Summary when to use tener que or deber in Spanish

We have seen it's not too different from the English use. Here just a short summary to keep in mind.

  Summary
1.

Tener que is used when there is no degree of freedom of choice, if there is no alternative for the person to act then tener que is the right choice. Even if the request comes from another person (and is not caused by nature or law) tener que is used when this person has the necessary authority to demand something without giving the chance to do something alternative. Tener que is also used when the logic says that there is no other choice.
Tienes que trabajar más, si quieres comprar el coche.
= You have to work more if you want to buy the car.
El juez dijo que tiene que irse a la cárcel.
= The judge says he has to go to jail.


2.

If it is about an attitude both tener que and deber can be used in Spanish.
Tienes que conocerlo, te va a fascinar.
Debes conocerlo, te va a fascinar.
= You have to meet her, she will fascinate you!!
= You should meet her, she will fascinate you!


3.

deber is used when there is an alternative and the acting person has room for manoeuvre.
Debe ir a España, pero creo que se va a Italia.
= He should go to Spain but I think he goes to Italy.


4.

deber is used with moral principles
Debes venerar a tus padres, pero si te pegan, debes defenderte.
= You should honour your parents but when they hit you, you should defend yourself.


5.

conjuntivo is used of both verbs when assuming something.
Tendría que saberlo.
= She should know it.


6.

In negative clauses deber can't be substituted with tener que (neither in English!).


The difference between the English use of shall/should and to have to in opposite to deber and tener que is minimal. Deber is used in indicativo and in conjuntivo (see chapter 20.7), shall is mostly used in the conjunctive form should. Tener que is used in indicativo and conjuntivo (again see chapter 20.7), to have to or also to must is only used in indicativo. Tener que is the unavoidable, deber expresses requirements, demands, attitudes, assumptions and suggestions.

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