Rule reminder

Impersonal expressions of necessity ranked by register: 'bisogna + infinitive' (neutral, everyday), 'occorre + infinitive' (formal, written), 'è necessario + infinitive' (formal/academic), 'si richiede + noun / che + subj' (official, bureaucratic), 'è d'obbligo + infinitive' (very formal), 'è indispensabile + infinitive / che + subj' (emphatic/academic). All except 'si richiede' take the infinitive when no subject follows. 'Si richiede che' takes the subjunctive.