Obligation and necessity
English uses 'have to / must / should / need to' somewhat interchangeably. Spanish has three different constructions for those shades: tener que (personal obligation), hay que (general rule), deber (moral duty). All three are followed by an infinitive.
TENER QUE — personal 'I have to'
tener (conjugated) + que + INFINITIVE. Tengo que estudiar = I have to study / I need to study. For any personal task: work, school, doctor's appointment. Tienes que comer = you need to eat. Tenemos que trabajar = we have to work.
HAY QUE — impersonal 'one must'
hay (from haber) + que + INFINITIVE. Doesn't change form. Hay que estudiar = one must study (anyone, in general). Hay que respetar las reglas = one must respect the rules. For general advice, rules, or universal truths.
DEBER — 'should / ought to' (moral)
deber (regular -er) + INFINITIVE. Debo ayudar a mi mamá = I should help my mom. Debes decir la verdad = you must tell the truth. Stronger than tener que — carries a sense of moral duty. The softer 'should advise' form is debería, but that's B1 territory.
PODER — 'can / be able to'
Goes with this group because it pairs with obligation. poder (irregular, o→ue): yo puedo, tú puedes, él puede, nosotros podemos, vosotros podéis, ellos pueden. + INFINITIVE. Puedo hablar español = I can speak Spanish. ¿Puedes ayudar? = can you help? No puedo = I can't.
QUERER — 'to want'
querer (irregular, e→ie): yo quiero, tú quieres, él quiere, nosotros queremos, vosotros queréis, ellos quieren. + INFINITIVE. Quiero comer = I want to eat. ¿Quieres café? = do you want coffee? All five — tener que, hay que, deber, poder, querer — take an INFINITIVE after them. Same pattern.
