可以引导定语从句。who引导定语从句作句子的主语、宾语。定语从句由关系代词who、whom、whose、which、what、as和关系副词where、when、why等引导。who引导的定语从句的先行词是表示人的名词或代词。
疑问代词who的用法
1.who在句中作主语：
Who's there?
who作主语通常视为单数，谓语动词要用单数。
改错题： Who are not at school today?改为is
其他主语例句： Who came when I was out?
Who put the light out before I finished my work?
2. who在句中作宾语：
Who have you chosen to take part in the sports meet?
你选了谁参加运动会？
此句重点是：主语是you，谓语动词用have，不能把who当主语，谓语用has就错了。
其他例句：
Who did she lend the bike to?
Who did you see then?
3.who在句中作表语：
Who is she?
Who are this girl and this boy?
this girl and this boy是两个人并列作主语，谓语动词用复数。
who用作疑问代词时，意思是“谁”，一般只用来指人,在句中用作宾语或主语。
I wonder who that letter was from.
我不知道是谁来的信。
who用作限制性或非限制性的关系代词，意思是“…的人，那个…”“他，她，他们”,在句中作主语。如：
The people (who) we met in France have sent us a card.
我们在法国结识的人给我们寄来了一张贺卡。