如果你在黑板上写字，下课时要擦掉。

If you write on the blackboard, rub it off at the end of the lesson.

《牛津词典》

你记得它的用途或者当老师在黑板上写字时它发出的声音吗？

Are you remembering what it is used for, or the sound it makes when the teacher writes on the blackboard?

youdao

粉笔是用来在黑板上写字的。

Chalks are used to write on the blackboard.

youdao

I can write on the board with it.

我能拿它在黑板上写字

耶鲁公开课 - 死亡课程节选