Tengo que ir a la ciudad hoy.
(a) When must is followed by a passive infinitive, hay que or ha de is used with the infinitive. Hay que also translates the impersonal one (we, you, they, people) must.
It mustn't be said that this meeting was in vain.
No ha de decirse que este encuentro fué en vano.
—EMILIA PARDO BAZÁN.
One must talk of that. One must remember that.
Hay que hablar de eso. Hay que acordarse de eso.
—FERNÁN CABALLERO.
(b) Expressing conjecture or probability must is often translated by the future and conditional of the verb which in English is the complementary infinitive with must. The future translates the present, and the conditional the perfect infinitive.
He must know something about it. He must have worn something.