Tranquilly means in a calm, quiet, peaceful way. It describes how something is done or how it happens — a river flowing tranquilly, someone sleeping tranquilly, a village sitting tranquilly at the foot of a hill. The calm here is a settled, unhurried calm, the kind you notice in a place or a scene, not the calm of a person forcing themselves to stay cool in a crisis. It is a literary word. You will meet it in novels, travel writing and poetry far more than in speech; if you say it in conversation it sounds a little grand. In ordinary talk people say 'peacefully', 'quietly' or 'calmly' instead. Note also that it is much less common than the adjective 'tranquil' — writers usually rearrange the sentence to use the adjective rather than reach for the adverb.

安詳地、寧靜地（書面語，口語會說「很平靜地」）