1．在限制性定语从句中，在介词不提前的情况下，若先行词是物，关系代词可用which，也可用that，且都可以省略；若先行词是人，关系代词可用who，whom，that或省略。如：
Do you know where the room (which/that) he once lived in is?
He is the film star (who/whom/that) the newspaper wrote an article about.
Here is the money (which/that) you can buy a piano with.
