The girl who I met yesterday is a student.为什么不用whom呢.I knew the person whom I saw yesterday.又为什么要用whom呢.

The girl who I met yesterday is a student.为什么不用whom呢.I knew the person whom I saw yesterday.又为什么要用whom呢.
The girl who I met yesterday is a student.
为什么不用whom呢.
I knew the person whom I saw yesterday.
又为什么要用whom呢.

The girl who I met yesterday is a student.为什么不用whom呢.I knew the person whom I saw yesterday.又为什么要用whom呢.
Who可以做主语也能做宾语,whom只做宾语

前者在句子中做主语、
后者做宾语

口语中可以用who