He is a good person who would like to help anyone. 他是一个好人，肯帮助任何人。

其中person就是先行词，who为关系代词

who是用来指代这个先行词person，因此称为关系代词。此外，who只能做“人”关系代词，比如boy、girl、people等。

除了who之外，关系代词还有whom、whose、which、that

其中，当定语从句修饰一个“人”时，选择关系代词须了解先行词在从句中的位置才能做出决定，可以用拆分复合句的方法来判断。例： 

1.He is a good person who would like to help anyone. 他是一个好人，肯帮助任何人。

拆分后：

He is a good person.

The person would like to help anyone.

拆分后可以发现，先行词在第二句中做主语，因此用who.

2.He is the man whom I met yesterday.

拆分后：

He is the man.

I met the man yesterday.

拆分后可以发现，先行词在第二句中做宾语,因此用whom.

当需要使用修饰“物”的关系代词时，例句是这样的：

1.This is a car which(that) is red.

拆分后:

This is a car.

The car is red

拆分后发现，先行词在第二句中做主语.

2.This is the book which you gave me yesterday.

拆分后：

This is the book.

You gave me the book yesterday

拆分后发现，先行词在第二句中做宾语

3.This is the room in which you were born.

拆分后：

This is the room.

You were born in the room

拆分后发现，先行词在第二句中做宾语。特别注意，当第二句中动词之后跟一个介词，在写定语从句时必须把介词提到关系代词之前。

最后一个，当关系代词要指代“人”或“物”的所有格(属格)时，例句是这样的：

This is the bike whose brake was damaged(the brake of which was damaged) 这辆自行车的刹车坏了。

拆分后：

This is the bike.

The bike‘s brake was damaged.或The brake of the bike was damaged.

先行词The bike在第二句中做定语。