deber

One of Spanish's core modal verbs, with two distinct uses: deber + infinitive expresses obligation ("must/should," debes estudiar), while deber + noun means to owe (le debo dinero). Deber de + infinitive, less common today, expresses probability (debe de ser tarde, "it must be late") rather than obligation.

• Debes terminar la tarea antes de salir. — You must finish your homework before going out.
• Le debo cien dólares a mi hermano. — I owe my brother a hundred dollars.
• Deben de ser las diez ya. — It must be ten o'clock by now.

Learners often mix up deber (obligation, "must") with tener que (also obligation, but slightly more urgent/circumstantial) and deber de (probability) — using deber de for obligation is a common but understandable slip, since many native speakers blur that distinction too.

Often confused with tener.