It is one of the best pictures which have been sold 这里which 为什么不对
正确的是：It is one of the best pictures that have been sold .
因为先行词 pictures由最高级the best修饰,所以关系代词只能用that.同时关系代词that在定语从句中做主语.
The day which she had to leave arrived at last. 这里which 为什么也不对
正确的是：The day when she had to leave arrived at last.
因为关系副词when= on the day,关系副词when 在定语从句中作状语.
that和which的区别是：
限制性定语从句中只能用that 引导定语从句的情况
1．当先行词是everything, anything, nothing (something 除外), all, none, few, little, some等代词时,或当先行词受every, any, all, some, no, little, few, much等代词修饰时.如：
Have you taken down everything that Mr. Li said? 李老师讲的你都记下来了吗?
There seems to be nothing that is impossible to him in the world.
对他来说似乎世界上没有什么不可能的事.
All that can bedone has been done. 所有能做的都做好了.
There is little that I can do for you. 我不能为你干什么.
He stayed in the library and looked up any information that they needed.
他呆在图书馆查找所需的资料.
注意：当先行词指人时,偶尔也可用关系代词who,如：
Any man that / who has a sense of duty won’t do such a thing.
任何有责任感的人都不会做这样的事.
All the guests that / who were invited to her wedding were important people.
所有应邀来参加她婚礼的客人都是重要人物.
2．当先行词被序数词修饰时.如：
The first place that they visited in London was the Big Ben.
在伦敦他们参观的第一个地方是大本钟.
3．当先行词被形容词最高级修饰时.如：
This is the best film that I have ever seen. 这是我看过的最好的电影.
4．当先行词被the very, the only修饰时.如：
This is the very dictionary that I want to buy. 这正是我要买的词典.
After the fire in his house, the old car is the only thing that he owns. 家里发生了火灾过后,那辆旧车成了他的唯一拥有.
注意：当先行词指人时,偶尔也可用关系代词who.如：
Wang Hua is the only person in our school who will attend the meeting. 王华是我校唯一出席会议的人.
5．当先行词前面有who, which等疑问代词时.如：
Who is the man that is standing by the gate? 站在门口的那个人是谁?
Which is theT-shirt that fits me most? 哪件T恤衫最合我的身?
6．当先行词为人与动物或人与物时.如：
They talked about the persons and things that they remembered at school 他们谈论着他们所能记起的在校时的人和事.
Look at the man and his donkey that are walking up the street. 瞧瞧那个沿街走来的人和他的毛驴.
非限制性定语从句通常不能用that引导.如：
1. I likethe book, which was bought yesterday. 我喜欢这本书,这是昨天买的.
2. I like the book which /that was bought yesterday. 我喜欢昨天买的那本书.
“介词+关系代词”引导的定语从句
关系代词在定语从句中作介词宾语时,从句常常由“介词+关系代词which”引出.
The school (which / that) he once studiedin is very famous.
The school in which he once studied is veryfamous. 他曾经就读过的学校很出名.