先行词是人,且在从句中作主语时，用关系代词(Relative Pronoun) who/that;在从句中定语从句的引导词作宾语时，用who/whom/that,或者省略
The man who/that lives in that house is my uncle． 住在那座房子里的人是我叔叔。
That is the girl （whom/who/that) I met at the party yesterday． 那个就是我昨天在晚会上遇到的女孩。
先行词是物，可用关系代词whiclvthat 充当从句中的主语和宾语，作宾语时可省略。which 除了指物以外还可以指婴儿、动物和表示单数意义的集合名词，此外还可以指人的属性，如身份、地位、职业或品格等
My favorite place is Kunming which/that is called as the Spring City. 我最喜欢的地方就是被称为“春城”的昆明。
This is the place (which/that） he would like to visit. 这就是他想去游览的地方。
先行词是人或物，且在从句中作定语时，用whose 引导,相当于先行词的所有格，whose 也可以替换成 ofwhich(指物）或 of whom(指人）
You're the only one whose (=of whom) advice he might listen to. 只有你的建议他可能会听。
They live in a room whose (=of which) window opens to the south. 他们住在一间窗户朝南开的房间里
先行词是时间、地点、原因，且在从句中作状语时，分别用关系副词(Relative Adverb) when, where, why引导。when 和where 可以用“介词+which”替换；why 的先行词只能是reason,why 可用“for which"代替
She remembered the day when (= on which) Paul had first walked into her office. 她记得保罗第一次走进她办公室的那一天。
We then moved to Paris, where (= in which) we lived for six years. 我们后来搬到巴黎，在那里住了6年。
只能用that, 不用which 引导定语从向的特殊情况
当先行词是 all, everything, nothing, something, anything, little, none, few 等不定代词时
All that we need is the support from you. 我们所要的一切就是你的支持。
当先行词被序数词或形容词最高级修饰时
The first lesson that I leared will never be forgotten. 我永远不会忘记我的第一堂课。
当先行词被 no, every, some, any, only, very，much 等限定词修饰时
He is the only person that I want to see. 他是唯一一个我想见的人。
当先行词既有人又有物时
The passengers and parcels that filled the car were mixed. 满车的旅客和行李混在一起了。
当先行词为主句的表语或关系代词为从句的表语时
Our school is no longer the school theat it used to be. 我们学校不再是曾经的那所学校了。
there be 句型中,先行词为物时
There is a room in the building that is still free. 这栋楼里有间房仍然空着
当先行词前面有who, which,what 等特殊代词时
Who is the lady that is waiting at the school gate? 在校门口等侯的那位女士是谁？
只能用 which，不能用that 引1导定语从句的特殊情况
which 可以引导非限制性定语从句,that 则不能
He changed his mind, which made me very angry. 他改变了主意，这使我很生气。
which 之前可以接介词,that 则不能
The questions were all on opera, about which I know nothing．那些问题都是关于歌剧的，我对歌剧一无所知。
当先行词本身为 that 时
What's that wohich flashed in the sky just now？刚刚在空中闪现的是什么？
准关系代词 as, than, but
as, than, but 这几个连接词可以像关系词一样,前面有名词作先行词;但它们后面的定语从句却不像典型的定语从句。所以这些既像连接词又像关系词的词，我们就称为准关系代词。准关系代词只有than,as, but, 分别用在以下情况
as 主要用在“such （+n.)+as..”（和…同样的），"the same （+n）+as..”（像…⋯这样的），the same as(同样的人或事)，such as(这样的东西或人），as.. as 等结构中
She has as much money as is required. 她要多少钱就有多少钱。(作主语）
than 作为准关系代词出现在 more. than, less.. than, fewer. than 等结构中，这时 than 前 面要有名词
I have more money than is needed. 我的钱比需要的多。
but 是具有否定感义的准关系代词，代替人或物，在意义上等同于“that which/who. not”,but 的面的主句要有另一个否定意义
There is no one but is concerned about his future. 没有人不关心自己的未来
