在什么情况下关系代词作宾语不能省略

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.