See -er verbs.

e.g.,

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)