I定語從句
1．先行詞為all， anything， something， nothing， everything， much， little， none等不定代詞時，關係代詞一般只用that，不用which。在大多數情況下that可以省略．
Please tell me everything （that） you know about the matter.
That’s all （that） we can do at the moment.
2．as引出的限制性定語從句
在such … as的結構中as可作關係代詞，引出限制性定語從句。有時和same連用，在從句中可作主語、賓語或表語等。
Such people as were recommended by him were reliable.
I’ve never seen such a talented young man as he is.
I have the same trouble as you （have）。
3．as引出的非限制性定語從句
as可作關係代詞引出非限制性定語從句，代替整個主句，通常譯為“（正）如…一樣”，“（正）象…一樣”等。as引導的從句一般用逗號與主句隔開，可以位於主句的前面、中間或後面．
I live a long way from work， as you know.
She did not， as her friend had feared， break down.
As is generally accepted， economic growth is determined by the smooth development of production.
4．分隔式定語從句
定語從句一般緊跟在先行詞之後，但有時會被其他句子成分與先行詞隔開，從而構成分隔式定語從句。
The days are gone when power politics worked.
A new teacher will come tomorrow who will teach you French.
5．介詞+關係代詞（which/whom等）引出的定語從句
如果關係代詞（which/whom等）在定語從句中作介詞的賓語，那麼這個介詞可以提到從句前，構成“介詞+關係代詞（which/whom等）+定語從句”。在這種結構中關係代詞不可以用that。
This is a subject about which we might argue for a long while.
The four travelers with whom I shared the room were pleasant people.
6．關係代詞在限制性定語從句中常可以省略，一般有以下幾種情況：
1） 關係代詞在從句中作直接賓語時，在大多數情況下可以省略。
This is something （that） you must always keep in mind.
The man （whom） you just met is our manager.
關係代詞作介詞賓語時，介詞位於句尾時關係代詞可以省略；介詞位於關係代詞之前時則關係代詞不能省略。
This is the room （which） Churchill was born in.
This is the room in which Churchill was born.（which不可省略）
2） 關係代詞作主語時一般不能省略，但如果關係代詞在由there … be存在句構成的.定語從句中作主語時常可省略。
This is the only book （that） there is on this subject.
The old professor made full use of the time （that） there was left to him to continue his research.
3） 在way後面的定語從句中in which或that通常省略。
That’s the way （that/in which） I look at it.
I don’t like the way （that/in which） you laugh at her.
7．非限制性定語從句
非限制性定語從句修飾先行詞或整個主句，起補充說明的作用，如果省略掉，原句句意仍保持完整。非限制性定語從句要用逗號與主句隔開，其引導詞不能用that。非限制性定語從句屬於正式語體。經常考到的非限制性定語從句有以下三種。
1）由which、as引導的非限制性定語從句。which、as代表整個主句。
He said that he had never seen her before， which was not true.
China is still a developing country， which is known to all of us.
2）由 “介詞+關係代詞”引導的非限制性定語從句。
He sent her a letter， in which he said that he was sorry for what he had done to her.
He failed to pass the exam， because of which his parents scolded him.
4） 由 “數詞、代詞或名詞+of +關係代詞”引導的非限制性定語從句。
He had three sons， one of whom was my son’s classmate.
There are about twenty students in this course， most of whom are freshmen.
They are two different words， the spellings of which are easily confused.
