He doesn't know next 1how to do 2what to do next 3how to do it

问题描述:

He doesn't know next 1how to do 2what to do next 3how to do it

答案是:3.
因为在这里,what是代词,可以单独作do的宾语,所以可以直接说: what to do next.
但how 是副词,不能作do的宾语,所以要添加it 作do的宾语.