（一）定语从句在下列情况下只能用that,而不能用which指物.（指人时可以用who/whom）
1.当先行词被形容词最高级或者序数词修饰时.如：
This is the best film that I have ever seen.这是我看过的最好的电影.
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.
2.当先行词是不定代词everything,anything,nothing ,something,all,none,few,little,some,much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 be done 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.
3.当先行词被the very,the only,the same,the last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.
4.当先行词是who,或which引导的主句时.如：
Who is the man that is standing by the gate?站在门口的那个人是谁?
Which is the T-shirt that fits me most?哪件T恤衫最合我的身?
5.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.
什么时候只能which 而不能用that呢?1.在非限定性定语从句中；2.在介词之后.