①Correct. 「청소하기 전에」 = before cleaning. You must turn the power off (전원을 끄다) first, and only then clean the machine — a running machine is dangerous to clean.
②「청소하고 나서」 = after cleaning. That is the wrong order: you would clean the machine while the power is still on. The power must go off before cleaning.
③「청소하는 동안」 = while cleaning. The power has to be off from the start, not turned off in the middle of cleaning.
④「청소한 다음에」 = after cleaning — the same wrong order as 「-고 나서」. Turning the power off comes before cleaning, not after.
非官方練習教材。Soho Korean 與 HRD Korea(Human Resources Development Service of Korea)、NIIED(National Institute for International Education)及韓國雇用勞動部(Ministry of Employment and Labor)沒有任何隸屬、背書或合作關係。EPS-TOPIK 和 TOPIK 是這些機關舉辦的考試名稱;這裡的題目全部由 Soho Korean 撰寫。