① 主 语：
I don't know who helped me.
② 宾 语 ( = whom )：
I don't know who / whom he likes.
紧跟在介词的后边做宾语时，只能用 whom
对比下面两个句子：
I don't know who / whom he will work with.
I don't know with whom he will work.
③ 表 语
I don't know who he is
who可以在定语从句、宾语从句等中作宾语。
例句：He is the man who we are waiting for.他就是那个我们等待的人。
需要注意的是，当宾语从句的连接词who在从句中作宾语时，可以改为宾格形式whom，也可以用主格形式who。
例如：I wonder who she dislikes.=I wonder whom she dislikes.我想知道她讨厌谁。
who 在从句中充当宾语的时候可以省略，且who的前面不能有介词。whom 在从句中充当不能充当主语，只能充当宾语，前面可以有介词。
如果宾语从句的引导词是who，那就是。问的是人。比如， I want to know Who is the tallest in your class。我想知道你们班谁个子最高。
