Who can write some words on the wall?（Who替代主语My sister)
What can my/your sister write on the wall?(What 替代宾语some words)
Where can my/your sister write some words?(Where替代地点状语on the wall)
What can my/your sister write some words on?(What 替代地点状语的名词部分the wall)
Wh-疑问词都是替代句中不同的部分,改句时,疑问词打头,然后除了替代主语的Who后面直接跟原句原顺序外,其他疑问词后面要把can提前,然后再跟剩余部分（被替代部分去掉）.