"The bird quietly writes a letter."
Translation:새가 조용히 편지를 써요.
Most Sino-Korean -하다 adjectives have 2 adverbs, the regular adverb -히 (English '-ly' adverbs) which modifies/describes the action (represented by the verb); and the base adverb -하게 (which modifies/describes the temporary state adopted by the agent/subject). Base adverbs do not always translate into English as adverbs. They tend to preserve their adjective forms.
새가 조용히 편지를 씁니다 (regular adverb) = The bird writes the letter quietly [ --> 'quietly' describes the way the letter writing is done ]
새가 조용하게 편지를 씁니다 = The bird quiet (in a quiet manner/calmly) writes the letter [ --> 'quiet', adverb describing the temporary state of the agent i.e. bird when it writes the letter ]
A very helpful example from @susieowly :
마을은 조용히 변했다 (regular adverb) = The village has quietly changed. (<- the change is quiet, subtle)
마을은 조용하게 변했다 (base adverb) = The village turned quiet (base adverb acting on the agent i.e. the village) [<- The village is now in a temporary state of quietness].