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Expressing Obligation:

There are basically FOUR ways in which we express obligation in English and in Spanish:

            a.) the obligation of something which is external to ourselves (as in, "I have to...").

            b.) the obligation of something which is morally imposed ( as in "I shouldn´t smoke so much").

            c.) the obligation of something which we feel we are supposed to do (as in, "I´m supposed to go to a party tonight").

            d.) the obligation of something which applies to everyone (generally) and which is impersonal in its character (as
                   in, one must eat a healthy diet.

Let´s look at some examples.

In the first instance, we express obligation using "tener que" plus an infinitive of another verb.

            I have to go to the doctor, because I have an infection.
            Tengo que ir al medico porque tengo una infección.

            I have to study tonight, because there´s a test tomorrow.
            Tengo que estudiar esta noche porque hay un examen mañana.

In the second instance, we express moral obligation using "deber" plus an infinive of another verb.

            I shouldn´t smoke so much.
            No debo fumar tanto.

            You really ought not to swear so much.
            No debes usar las palabrotas tanto.

In the third instance, we express the supposition of an obligation by using "he de" plus the infinitive of another verb.

            I am supposed to go to Margaret´s party tonight.
            He de asistir a la fiesta de Margarita esta noche.

            He is supposed to come for his interview tomorrow.
            Ha de venir para hacer su entrevista mañana.

Finally, in the fourth instance, we express impersonal obligation by the use of "hay que" plus an infinitive of another verb.

            Its necessary to enter through the main entrance.
            Hay que entrar por la entrada principal.

            One must work to eat.
            Hay que trabajar para comer.


Unlike English, which requires the use of defective verbs, e.g., should, ought, must, and which exist only in certain tenses,
obligation in Spanish is expressed clearly and completely by the above forms and such obligation can be expressed in all tenses.