①that在定语从句中作主语时不省略，作宾语时可以省略；
②that不引导非限制性定语从句；
③that前不加介词。
who的先行词是人。who在定语从句中作主语、宾语或表语。
先行词为one、ones、anyone或 those时，或双重定语从句中一个定语从句的关系代词是that，另一个指人的关系代词词用who。
关系构成
关系从句的句法功能主要是充当定语。在英语中，关系从句通常位于它所修饰的词（组）之后。被关系从句修饰的词（组）叫做先行词（英语：antecedent），引导关系从句的词称为关系词，关系词指代先行词并在关系从句中充当一定的成分。
例如：This is the book which interests me（“which interests me”是关系从句，修饰先行词“book”，而关系词“which”指代先行词“book”并在关系子句中充当主语。这句话可以拆分为两个句子来理解：“This is the book.”和“The book interests me”）
以上内容参考：百度百科-定语从句
who,that在很多情况下可以通用,但是有些情况只能用who:
1.先行词是one,ones,anyone的时候宜用who.
Anyone who fails to finish the task given should be criticized.
2.先行词为those的时候,宜用who为关系代词
Those who want to go to the Great Wall sign up here.
3.当先行词有比较长的后置定语的时候
I met a foreigner in the street yesterday who could asked me questions in
Chinese.
4.一个句子中带有两个定语从句,其中一个定语从句的关系代词为that,另一个则用
who.
The boy that you met last night is the group leader who studies very hard.
5.在there be 开头的句子中
There is an old man who wants to see you.
that常用作关系代词，可指代某人，也可指代某物，指物时有的时候可用which替换that，指人时可用who替换。但在下列情况下，一般用that。
一、that指代某物事时
1.先行词为all, few, little, much, the one, something, anything, everything, nothing等时。如：
We'll do all that we can to protect the endangered plants and animals.
2. 先行词被all, every, no, some, any, (a)few, (a)little, much等限定词修饰时。如：
You can borrow any book that you want to read in our school library
3. 先行词被序数词或形容词最高级修饰时。如：
This is the most beautiful city that I've ever seen.
4. 先行词被the only, the very, the right, the last 等修饰时。如：
This is the very factory that they visited last summer holiday.
5. 先行词是并列结构，既有人又有物时。如：
He talked happily of the writer and his books that interested him.
6. 先行词前有the same 修饰，表示和先行词是同一物时。如：
This is the same purse that I lost yesterday
7. 先行词为数词时。
Look at the books on the bookshelf. You can see the two that you bought me for my birthday.
8. 如果which引导的定语从句中又含有一个定语从句，为避免重复，第二个定语从句用that。如：
They built a factory which produced things that had never been seen.
9. 以which作主语开头的特殊问句，为了避免重复，定语从句用that。如。
Which is the bus that you will take?
10. 关系代词在定语从句中作表语时，而且通常省略。如：
My hometown is no longer the place that it used to be.
11. 关系代词在there be 句型中作主语时,而且常可以省略。如：
This is the fastest train (that) there has ever been.
二、that 指代某人时。
1. 泛指某人时。如：
He is a man that is never at a loss.
2. 主句是以作主语的who开头的特殊问句，为了避免重复时。如：
Who is the person that is talking with our headmaster?
3. 先行词前有the same时。如：
This is the same man that gave us a talk last year.
4. 关系代词在定语从句中作表语时。如：
He was not the man (that) he was 10 years ago.
另外，that也可用作关系副词，表示时间、地点、原因和方式，在口语中可以用来代替when, where, why 引导的定语从句。在以下名词day, year, time, moment, reason, place等作先行词时，可用that作关系副词引导定语从句。如：
I'll never forget the day (that) I joined the League.
This is the first time that I have been abroad for traveling.
（注：先行词是time,前面有序数词或last修饰时，常用that引导定语从句或者省略。）
当先行词为表示方式的词the way时，可用that引导定语从句, that常可以省略。
I don't the way you speak to her.