devoir

Covers both obligation ("must / have to") and probability ("must be"), depending on context — je dois partir ("I have to leave") versus il doit être tard ("it must be late"). Also a standalone verb meaning "to owe" (je te dois dix euros).

• Je dois finir ce rapport. — I have to finish this report.
• Tu devrais te reposer. — You should rest.
• Elle doit être fatiguée. — She must be tired.

Confusing the present (je dois, a real obligation right now) with the conditional (je devrais, softer — "I should"). Devoir in the conditional is advice; devoir in the present is a near-command, and the two get swapped constantly by beginners.

• politics Le Devoir — The Montreal newspaper founded in 1910 by Henri Bourassa — its name is the noun form of devoir ("duty") — historically a leading voice for Québécois nationalism and still publishing today.
• expression Devoir de mémoire — "Duty to remember" — a phrase that took hold in French public debate in the 1990s, especially around official recognition of France's role in the Holocaust deportations.