deber, tener que, hay que: obligation, advice and deduction
The most common way to express obligation is tener que + infinitive. It conjugates like tener and works in every tense.
Tengo que estudiar esta noche. (I have to study tonight.)
Tuvimos que esperar dos horas. (We had to wait two hours.)
Tener que expresses external, practical obligation — deadlines, rules, circumstances.
Deber + infinitive expresses moral duty or strong advice. In the conditional (debería) it softens to English "should".
Debes descansar más. (You must rest more.)
Deberías hablar con ella. (You should talk to her.)
Debería is the polite adviser's form — perfect for giving recommendations in writing exams.
Add de and the meaning changes completely: deber de + infinitive = deduction ("must be" in the sense of probability).
Deben de ser las diez. (It must be ten o'clock — I deduce.)
Debe de estar cansado. (He must be tired — guessing.)
Compare: Debe estudiar (he must study — duty) vs Debe de estudiar mucho (he must study a lot — I deduce it from his grades).
Hay que + infinitive states what "one must" do — no subject, totally impersonal. English has no exact equivalent; the closest is "you have to / it is necessary to".
Hay que reservar con antelación. (You have to book in advance.)
Hay que tener paciencia. (One must be patient.)
Use hay que for rules and general advice; use tengo que when the obligation is yours personally.
Careful — the negatives don't map onto English neatly.
No debes fumar aquí. (You mustn't smoke here — prohibition.)
No tienes que venir. (You don't have to come — no obligation, it's optional!)
No hay que pagar entrada. (You don't need to pay an entrance fee.)
English speakers' classic trap: no tener que = absence of obligation, NOT prohibition. "You mustn't" = no debes.
Traps for English speakers
These are the errors English speakers make most often.