關系副詞引導的定語從句
關系副詞可代替的先行詞是時間、地點或理由的名詞，在從句中作狀語。
1. 關系副詞when， where， why的含義相當于"介詞 which"結構，因此常常和"介詞 which" 結構交替使用，例如：
There are occasions when (on which) one must yield.
Beijing is the place where (in which) I was born.
Is this the reason why (for which) he refused our offer？
限制性和非限制性定語從句 (***)
例如：
1)This is the house which we bought last month.(限制性)
The house， which we bought last month， is very nice.。(非限制性)
2) 當先行詞是專有名詞或物主代詞和指示代詞所修飾時，其后的定語從句通常是非限制性的，例如：
Charles Smith， who was my former teacher， retired last year.
My house， which I bought last year， has got a lovely garden.
This novel， which I have read three times， is very touching.
3) 非限制性定語從句還能將整個主句作為先行詞， 對其進行修飾， 這時從句謂語動詞要用第三人稱單數，例如：
He seems not to have grasped what I meant， which greatly upsets me.
Liquid water changes to vapor， which is called evaporation.
說明：關系代詞that和關系副詞why不能引導非限制性定語從句。
b. 名詞性從句(Noun Clauses)
在句子中起名詞作用的句子叫名詞性從句。名詞性從句的功能相當于名詞詞組，它能在復合句中作主語、賓語、表語、同位語等。根據它在句中不同的語法功能，名詞性從句分為主語從句、賓語從句、表語從句和同位語從句。在名詞性從句中，連接詞that既沒有實際意義，又不充當任何成分，只起引導從句的作用， 引導賓語從句時經常省去。
一、主語從句 (The Subject Clause)：在復合句中作主句的主語。為保持句子結構平衡，主語從句通常被置于句末，而用it作形式主語。
That light travels in straight line is known to all.
When the plane is to take off hasn’t been announced.
It was my fault that I had him play football yesterday afternoon.
It is important that we should go to the school to talk with the teacher.
It is still unknown which team will win the match.
二、賓語從句 (The Object Clause)：主要作及物動詞的賓語，有時置于介詞或形容詞之后。
He said (that) his father had come back from the United States.
He asked how much I paid for the violin.
No one can be sure what man will look like in a million years.
如果賓語從句帶有補足語，通常用it作形式賓語，而把賓語從句置于補足語之后。
I think it necessary that we should pay close attention to SARS.
We all feel it our duty that we should serve the people heart and soul.