être

The single most important verb in French — used for identity, description, and location ("je suis fatigué"), and as the auxiliary for a specific set of intransitive/movement verbs and all reflexive verbs in the passé composé. Almost every other tense and mood in this table leans on être being solid first.

• Je suis étudiante. — I am a student.
• Elle est arrivée hier soir. — She arrived last night.
• Nous sommes prêts. — We are ready.

English speakers default to avoir for every compound tense out of habit — but a fixed set of verbs (aller, venir, arriver, partir, and the other "Dr & Mrs Vandertramp" verbs, plus all reflexives) take être instead, and the past participle then has to agree with the subject.

• quote "Je pense, donc je suis" — Descartes' "Cogito ergo sum" ("I think, therefore I am") — one of the most famous lines in Western philosophy.
• book L'Être et le néant — Jean-Paul Sartre's 1943 philosophical work, translated into English as "Being and Nothingness."
• politics Je suis Charlie — The global solidarity slogan that spread after the 2015 Charlie Hebdo attack in Paris.
• film Être et avoir — Nicolas Philibert's 2002 documentary about a rural one-room French schoolhouse — still the highest-grossing French documentary of all time.
• expression "C'est la vie" — "C'est" (that/it is) is a contraction of ce + est, a conjugated form of être — this phrase has been borrowed into English almost unchanged.