什么情况下定语从句必须用who引导？

(1)当先行词是all,any,a few等不定代词指人时。
（2）先行词为that,this,these,thoes指人时。
（3）直接位于介词后作宾语时，用whom。

除此以外，没有特别规定！！
1.当先行词为one,ones(此处均指人),those,anyone,people时。如：
Those who are from Qingdao come this way.从青岛来的人走这条路。
2.以 There be 开头的句子。如：
There's a man at the door who wants to see you.在门边的那个人想见你。
3.当先行词后有较长的后置定语或定语从句被分开时。如:
A teacher will come tomorrow will teach you English.教你们英语的一个老师明天会来。
4.当一个句子带有两个定语从句,其中一个用that,另一个宜用who。如：
The student that was praised at yesterday's meeting is a monitor who is very modest.
5.在非限制性定语从句中。如：
The famous basketball star ,who is handsome attend a lot of attentions.
6.当先行词用body或one构成的不定代词时。如：
Is there anybody else who will provide us help?
7.当先行词是集合名词时，如果着眼于集体，使用that/which；如果着眼于个体，则使用who。如：
a.Our team ,who are all at the age of twenty,will do well in the game.
b.Our team,which took the second place last year,played better this year.

你好。比如说：
She is looking for the man. The man has taken her cellphone
-------- the man 被句子修饰，用who :

She is looking for the man WHO has taken her cellphone --------这里，当然也可以用THAT替换WHO.

如果修饰的是人的话