- 느린 (느리다: adj): slow. <=> 빠른 (빠르다): fast. - 느리게 (adv): slowly <=> 빨리, 빠르게: fast, rapidly. - 천천히 (adv): slowly, taking one's time <=> 빨리, 급히: fast, quickly, rushingly. * 천천히 only exists as an adverb. There is no adjectives like 천천하다 or 천천한. * 느린/느리게 works better for a constant rate (speed), while 천천히 is more for people's movements and actions. So the adjective 느린 and adverbs 느리게 and 천천히 have different functions in a sentence. Examples: - 이 기차는 너무 느리다 [빠르다]: This train is too slow [fast]. - 이 기차는 너무 느린 [빠른] 속도로 달린다: This train runs at too slow [fast] a speed. - 이 기차는 너무 느리게 [빠르게] 달린다: This train runs too slowly [fast]. - 음식은 천천히 먹어야 한다: You should eat taking your time. - 급히 서두를 필요 없다: There is no need to rush yourself too much.

They are almost same but have different nuance. 천천히 mean "consciously, carefully" 느리게 mean "slowly, sluggish"