The girl who is standing under the tree is my sister. The man whom you want to see has come.
1.定语从句的前面都有先行词(被修饰语) This is the boy whom we are looking for
4.关系词在定语从句中充当某一成分
2.定语从句应紧跟在先行词之后
3.先行词与定语从句之间要有关系词(有时可省略)