The passé antérieur is used after conjunctions of time — aussitôt que and dès que (as soon as), quand and lorsque (when), and après que (after) — to show that an action was completed before another action (expressed by the passé simple) took place.
The passé antérieur is the compound form of the passé simple and is formed by using the passé simple of the appropriate helping verb (avoir or être) + the past participle of the verb. Just like the passé simple, the passé antérieur is primarily used in formal and literary writings. Its English equivalent is “had” + past participle:
Aussitôt que le président eut signé le document, sa secrétaire l'emporta. (As soon as the president had signed the document, his secretary took it away.)
Dès que ses ennemis eurent envahi la France, Napoléon se retira à Elbe. (As soon as his enemies had invaded France, Napoleon retired to Elba.)