"Before class, the blackboard must be wiped" 或者 "The blackboard must be wiped before class" 是表达这个意思的常用英文表达方式。
这句话的来源可能是由于在英语中，"wipe" 是擦拭的意思，所以在擦黑板之前就会用这个动词来表示。
例句：
Please remember to wipe the blackboard before class.
请记得在课前擦黑板。
The teacher always makes sure the blackboard is wiped before the start of each lesson.
老师总是在每节课开始前确保黑板已经擦过。
The students were told to wipe the blackboard before they left the classroom.
学生们被告诉在离开教室之前擦黑板。
这句话的来源可能是由于在英语中，"wipe" 是擦拭的意思，所以在擦黑板之前就会用这个动词来表示。
例句：
Please remember to wipe the blackboard before class.
请记得在课前擦黑板。
The teacher always makes sure the blackboard is wiped before the start of each lesson.
老师总是在每节课开始前确保黑板已经擦过。
The students were told to wipe the blackboard before they left the classroom.
学生们被告诉在离开教室之前擦黑板。