She found a little girl who was crying. 为什么句中要用“who”?谁能补充点相关的语法知识?

问题描述:

She found a little girl who was crying. 为什么句中要用“who”?
谁能补充点相关的语法知识?

who was crying 是定语从句.可改此句为she found a little girl crying,