Who clean the blackboard yesterday?答案是John did.就是想问问为什么是did而不是cleaned?问题打错了是who cleaned
问题描述：
Who clean the blackboard yesterday?
答案是John did.就是想问问为什么是did而不是cleaned?
问题打错了是who cleaned
答
john did did就代指（cleaned the blackboard）
如果说john cleaned 就缺少一个宾语 如果说John cleaned the blackboard 又显得没John did那么简洁 所以就不说john cleaned