Say what you must, need to and should do
Spanish has three everyday structures for obligation. All are followed by an infinitive.
Tengo que estudiar. (I have to study.) — personal, most common
Debo estudiar. (I must study.) — stronger, more formal
Hay que estudiar. (One must study.) — impersonal, general rules
Tener que conjugates tener normally and adds que + infinitive. It is the everyday "have to".
Tengo que trabajar mañana. (I have to work tomorrow.)
Tienes que ver esta película. (You have to see this film.)
Tenemos que salir a las ocho. (We have to leave at eight.)
Never drop the que: tengo que ir, not "tengo ir".
Deber goes straight to the infinitive — no que. It sounds like a duty or strong recommendation.
Debes descansar más. (You must rest more.)
Los alumnos deben llegar puntuales. (Pupils must arrive on time.)
For advice — English "should" — use the conditional: deberías. Deberías dormir más. (You should sleep more.)
Hay que + infinitive states a general obligation with no particular subject — like English "you have to / one must".
Hay que reservar con antelación. (You have to book in advance.)
En España hay que cenar tarde. (In Spain one has to dine late.)
It never changes form — hay que works for everyone in general.
Careful with negatives: no tengo que means "I do not have to" (no obligation), while no debes means "you must not" (prohibition-ish advice).
No tienes que venir. (You do not have to come — optional.)
No debes fumar aquí. (You must not smoke here.)
Traps for English speakers
These are the errors English speakers make most often.