Idioms with Tener, Deber, and Haber

121. Tener que.  Tener que followed by the infinitive expresses obligation or necessity.

Tengo que estudiar mi lección. I have to study my lesson.
Teníamos mucho que hacer. We had much to do.
¿Tiene V. que ir ahora? Do you have to go now?

a. Moral obligation is more commonly expressed by deber (122).

b. Tener que may also denote mere provision.

No tenemos que comer. We have nothing to eat.

122. Deber.  Deber means primarily ' to owe.' From this are derived the ideas of necessity, obligation, duty.

Debo ir a ver a mi padre. I must go and see my father.
Debí estudiar ayer. I ought to have studied (it was my duty to study) yesterday.

a. Deber followed by the infinitive is also used to express probability. See also 103.

Debe estar malo. He is probably ill.

123. Haber de.  Haber de followed by an infinitive expresses (a) what is probably true in present time, (b) mere futurity, (c) volitional futurity.

a. Ha de ser muy rico. He must be very rich.
Ha de ser Carlos. It is probably Charles.
See 103 and 114.
 
b. Ha de venir mañana. He is coming to-morrow.
Han de dar un paseo. They are to take a walk.
 
c. He de castigarte si lo haces. I will punish you if you do it.
He de verla. I must see her (it is my intention to see her).

124. Haber que.   Haber que as an impersonal verb (see 114), followed by an infinitive, expresses also necessity, obligation, or mere futurity.

Hay mucho que estudiar. There is much to be studied.
Hay que hablar con cuidado.  One must speak with care.
Para llegar a tiempo hay que
ir aprisa.
To arrive on time we must hurry.

125. The Infinitive after Prepositions.

a. The infinitive is generally used in Spanish after a preposition where the present participle is used in English.

Vine sin verlo. I came without seeing him.
Tengo ganas de ir. I am desirous of going.
Después de comer, se fué. After eating, he went away.

b. ' On,' ' when,' ' as ' + a present participle is translated by al + the infinitive.

al llegar a la casa, on arriving (when we arrived, etc.) at the house.