(1)关系代词在定语从句中作宾语 She is the girl (whom / that) I loved before. (可以省略whom/that)(2)关系代词在定语从句中作表语He is no longer the boy (that) he used to be. (可以省略that)(3)关系代词作介词宾语，介词在从句句尾时可以省略Housing price is a problem (that/which) people are interested in.比较：Housing price is a problem in which people are interested.（此时只能用which且不能省略）
