Adverbs (부사) ~게, ~히, ~이

Adverbs (부사) are represented in Korean by three cute little markers: ~게, ~히, ~이.
The ~게 marker would be equivalent to the commonness of ~ly in English, so let's start there.

A: 앞머리는 어떨게 발라 드릴까요? How should I cut your bangs?
B: 조금 짧게 잘라 주세요 Just a little short (lit. Cut it a little shortly)

A: 이모, 사과 씻을까요? Aunt So-and-so, should I wash the apple?
B: 응 깨끗하게 씻어 주세요 yeah make it nice and clean (lit. Wash it cleanly)

A: 잘 먹겠습니다 Thanks for making the food (lit. I'll eat well)
B: 네 맛있게 드세요 Eat up! (lit. Eat it deliciously)

A: 주사를 놓아 드릴께요 I'm going to give you a shot, now.
B: 안 아프게 해주세요 Please be gentle (lit. Do it not hurtfully)

A: 선생님....아...그개...어 이잖아..... Teacher.... um, I uh....well, you see....
B: 응? 뭐라고? 크게 말해 Huh? What'd you say? Speak up (lit. speak loudly)

The "~히"and "~이" adverbs are practical however not nearly as common as the "~게" adverb and are specific to certain verbs. My best advice is to just listen to what is used more often for a given verb.

A: 와!! 왠 일이야? whoa... what are you doing here?
B: 쉿! 조용히! Shhh be quiet (lit. speak silently)
(*this is a situation where one can instead say 작게 말해야해 but it's not as common)

A: 엄마 다녀올께요! Mom, I'm going (to school and I'll be back later)!
B: 응 빨리 와 Come (home) quickly

A: 예들아! 사탕 줄까? Hey kids! Should I give you some candy?
B: 네! 많이 주세요! Yes! Give me a lot! (lit. many give me)