Susan is what is known as an addlehead. 该句不可以用who 替换 what,原因是,what 是融合性关系代词,其作用身兼两职,既是先行词,有是关系代词,what 等于a person who。而who 引导定语从句没有这个功能,它前面必须有先行词才可以。 Do you know who will come this afternoon? 这是 疑问代词 who 引导的间接疑问句,作know 的宾语。who 作了从句的主语。去掉Do you know,就是直接疑问句: Whowill come this afternoon?