stain /ste?n/ n. 污迹，污点；瑕疵
擦掉黑板上的字 rub words off the blackboard
A：Who rubbed my words off the blackboard?
谁把我写在黑板上的字擦掉了？
B：I did it. It's my duty today. Anything wrong?
我擦的。今天我值日。有什么问题吗？
duty /?dju?ti/ n. 责任；义务；职责；税
用水清洗黑板 wash the blackboard with water
A：There's so much chalk powder on the blackboard that we'd better wash the blackboard with water.
黑板上有这么多粉笔末，我们最好用水清洗一下黑板。
B：We didn't wipe down the blackboard last time, so we should do it carefully this time, I think.