See -er verbs.
te debo treinta euros – I owe you 30 euros
debemos respeto a Don Knuth – we owe respect to Don Knuth
deben de ser las cinco – it must be five o’clock
deber + infinitive – must do something
debo estudiar – I must study
debe tomar ... – you must take ...
See also tener.
tener que + inf is weaker than deber + inf (the
latter is unarguable, like shall in English standards-speak)