Learn French: Elle a fini brillamment, et le public a réagi bruyamment. - She finished brilliantly, and the audience reacted loudly.

It comes from the adjective brillant (brilliant or shining). For adjectives ending in -ant, we drop the -nt and add -mment to form the adverb. Notice that it is spelled with a double 'm'.

Why is bruyamment spelled with an 'a' (-amment) instead of an 'e' (-emment)?

Adverbs follow the spelling of the adjective they come from. Because the adjective for noisy or loud is bruyant (ending in -ant), its adverb form takes -amment. If the adjective ended in -ent, like prudent, the adverb would end in -emment (prudemment). Both endings are pronounced exactly the same way, sounding like a nasal 'ah' followed by 'mahn'.

Why do we use fini and réagi here?

These are the past participles of the verbs finir (to finish) and réagir (to react). Both are regular -ir verbs, which means you form their past participle simply by dropping the 'r'. They are paired with the helper verb avoir (a) to form the passé composé, which is the standard past tense used here for completed actions.

Why is le public translated as the audience?