Español — Obligation periphrases
Spanish expresses obligation with three periphrases plus an infinitive: Tengo que estudiar, Hay que esperar, Debes descansar. Adding de to deber changes everything — deber de expresses conjecture: Deben de ser las cinco.
How it works
Tener que, hay que and deber + infinitive all express obligation. Tener que and deber are personal — they agree with a subject: Tengo que salir, Debes estudiar. Hay que is impersonal, a general rule with no named subject: Hay que reservar. Only deber de + infinitive breaks the pattern: it states a conjecture, not a duty — Deben de ser las cinco means "it must be around five".
Forms and uses
| Periphrasis | Use | Example |
|---|---|---|
| tener que + inf. | Personal obligation or need | Tengo que estudiar. |
| hay que + inf. | Impersonal, general obligation | Hay que respetar las normas. |
| deber + inf. | Moral duty, advice | Debes descansar más. |
| deber de + inf. | Conjecture, probability (not obligation) | Deben de ser las cinco. |
| no tener que + inf. | Absence of obligation ("need not") | No tienes que venir. |
| no deber + inf. | Prohibition ("must not") | No debes fumar aquí. |
Good to know
The classic mistake is confusing deber and deber de. Debes estudiar is an obligation ("you must study"); Debes de estar cansado is a guess ("you must be tired"). In everyday speech people often drop the de and use deber for both, but in careful and written Spanish keep them apart. Also note: no tener que is not a prohibition — No tienes que venir means "you don't have to come", not "you must not come".