在下列四种情况下,关系代词虽作宾语,却不能省略。
一、在介词+whom /which结构中, whom, which不能省略。例如: Do you know any shops nearby in which I can buy flowers?你知道附近有可以买到花的商店吗? That is the headmaster with whom my father was talking just now.刚才和我爸爸交谈的那个人是校长。
二、在非限制性定语从句中,作宾语的关系代词也不能省略。例如: Mr Green, whom you know, is the tallest in our school. 格林先生是我们学校里最高的,你也认识他。 The elephant is like a spear, as anyone can see. 大象像长矛,任何人都知道。
三、在the same ... as, such ... as, as ... as, the same... that结构中, as, that 即使作宾语,也不能省略。例如: I have bought the same bike as you (have). (指同类用as)我买了一辆和你的一样的自行车。 This is the same book that I read the day before yesterday. 这就是我前天读过的那本书。(关系代词that如要换成as, 则指与我读的书一样,而不是同一本。)
四、当and, but, or等连词连接两个或两个以上的定语从句修饰一个先行词时,关系代词第一个可省略,第二、第三个等不可省略。例如: This is the book (which) I read yesterday and which I found very interesting. 这就是我昨天读的那本书,它很有趣。You have many people around you (whom) you always turn to and whom you will never forget.你身边有很多人,你经常向他们求助,你永远不要忘记他们。
参考资料：http://bbs.education.163.com/bbs/kaoyan/237480810.html