The verb deber and the verb phrase tener que are the two most common ways of expressing obligation in Spanish, to say that someone has to, should, ought or must do something. They are followed by the infinitive form of the verb.

A few examples:

• Tengo que ayudar en las reparaciones. Debo ayudar en las reparaciones. (I have to help with the repairs.)
• Tiene que comprar y añadir una nueva tarjeta prepagada de tiempo celular. Debe comprar y añadir una nueva tarjeta prepagada de tiempo celular. (You have to buy and add a new prepaid card for cellphone time.)

• Tenemos que estudiar la historia de Colombia. Debemos estudiar la historia de Colombia. (We should study the history of Colombia.)
• Tuvo que irse a trabajar. Debió irse a trabajar. (She had to go to work.)

As in the above examples, tener que and deber are usually interchangeable. However, tener que usually expresses a stronger sense of obligation than does deber.

Note that tener is conjugated irregularly. Deber, however, is conjugated regularly.

The phrase no tener más remedio que is one of the most common ways of expressing extremely strong obligation:

• No tiene más remedio que decir la verdad. (He absolutely has to tell the truth.)
• No me dejas otra alternativa y no tengo más remedio que aceptar. (You leave me no other choice, and I must accept.)

A weaker sense of obligation can be expressed by using the conditional form of deber. The conditional forms of deber are especially common in questions.

• ¿Por qué debería comprar un lavaplatos? (Why do I have to buy a dishwasher?)
• Deberíamos salir. (We need to get going.)

• Los economistas deberían concentrar su atención en los desempleados. (The economists should focus their attention on the unemployed.)

A vague sense of obligation can also be expressed by use of haber de, although it isn't used in all areas and can sound stuffy. Example: He de estar a dieta, I need to be on a diet.

Sometimes the verb necesitar is also used as equivalent of tener que or deber, although it is less common than the corresponding English verb, "to need":

• Necesito obtener certificación para trabajar. (I need to get certification in order to work.)
• Necesitas hablar de lo que te preocupa. (You need to talk about what's worrying you.)

Note: It is possible that you will hear native speakers substitute deber de for deber when expressing obligation. However, this use of deber is considered substandard by some grammarians and is probably best avoided by those who are learning the language. (The accepted way to use deber de is to express likelihood. Example: Debe de llover en Managua, it's probably raining in Managua.)