Confirm the instruction — say "Yes" only after you are sure.

다시 한 번 말해 주세요.

dasi han beon malhae juseyo.

The first thing to say when you don't understand an instruction. You can ask several times. (안 되겠어요)

천천히 말해 주세요.

cheoncheonhi malhae juseyo.

Use this when you can't keep up because the person is speaking too fast. It is short and polite, so it works anywhere. (말이 빨라서 못 따라갈 때.)

잘 모르겠습니다.

jal moreugetseumnida.

It is a hundred times safer to say you did not understand than to pretend you know. (이해 못 했다고 말하는 것이 아는 척보다 백 번 안전하다.)

제가 지금 이거 하면 됩니까?

jega jigeum igeo hamyeon doemnikka?

A phrase you say to confirm an instruction by repeating it out loud. When you point to what you're going to do and ask, it becomes clear.

알겠습니다. 확인했습니다.

algetseumnida. hwaginhaetseumnida.

Use it only when you really understand. If you say you understand but do something different, you will lose trust.

한 번 보여 주세요.

han beon boyeo juseyo.

If it is difficult to explain with words, ask for a demonstration. At the workplace, seeing it with your eyes is the fastest way.

어느 것입니까? 이거요?

eoneu geosimnikka? igeoyo?

When "이거," "저거," and "거기" are confusing, point to the actual object and confirm.