1. 先行词为all, anything, something, nothing, everything, much, little, none等不定代词时，关系代词一般只用that，不用which。在大多数情况下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.
在such … as的结构中as可作关系代词，引出限制性定语从句。有时和same连用，在从句中可作主语、宾语或表语等。
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).
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.
The days are gone when power politics worked.
A new teacher will come tomorrow who will teach you French.
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.
This is something (that) you must always keep in mind.
The man (whom) you just met is our manager.
This is the room (which) Churchill was born in.
This is the room in which Churchill was born.(which不可省略)
2) 关系代词作主语时一般不能省略，但如果关系代词在由there … be存在句构成的定语从句中作主语时常可省略。
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后面的定语从句中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.
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 “介词+关系代词”引导的非限制性定语从句。
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 “数词、代词或名词+of +关系代词”引导的非限制性定语从句。
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.
时间状语从句主要由以下连词引导：when, whenever, as, while, since, until, till, before, after, as soon as, once, hardly (scarcely) … when, no sooner … than。
I will discuss the matter with you when we meet tomorrow.
He didn’t go to sleep until he finished doing his homework.
You have changed a lot since we met last time.
时间状语从句还可由某些可充当连词的名词(the moment, the instant, every time,等)和副词(instantly, directly等)引导：
I will tell you the news the instant I know.
Directly the teacher came in everyone was quiet.
① before除了可表示 “在…之前”外，还可用来强调主句所表达的时间、距离很长或花费的力量相比较大，可译为“…才”。(“not … before” 可用来强调主句所表达的时间、距离很短或花费的力量相比较小，可译为“…就”)。
It was a long time before I got to sleep again.
They had not been married a month before they quarreled.
② 如果位于when引导的分句之前的主句使用过去进行时、过去完成时或“was/were about to, was/were on the point of”等结构时，when表示突然发生某事，可译为“正在…突然”或“刚…就”等。这时，不能够用as或while来代替when。
I was walking on the street when I ran into my old friend John.
She had just fallen asleep when the telephone rang.
We were about to leave when it began to rain.
条件状语从句主要由以下连词引导：if, unless, providing/provided (that), suppose/supposing (that), on condition (that), as/so long as, in case等。
So long as you work hard, you will surely achieve good results in examinations.
I shall return next Thursday unless something unexpected happens.
I will go provided that you go with me.
Please give this letter to John in case he comes.
让步状语从句主要由以下连词引导：though, although, even if, even though, whatever, however, whoever, wherever, no matter how/ what/ where/ when等。以下介绍几种较特殊的让步状语从句。
While I admit I did it, I didn’t intend to.
While I understand what you say, I can’t agree with you.
(2) 短语“even now/then/so”相当于“though it is/was true”，表示“尽管如此”，“尽管这样”。
The fire was out, but even so, the smell of smoke was strong.
I’ve tried my best, but even now/then she is not satisfied.
Child as he was, he could speak four languages.
Hard as she tried, she failed to get the job.
比较状语从句主要由以下连词引导：(not) as … as, (not) the same as, (not) such … as, not so … as, than, the + 比较级…, the +比较级…等。
We have had a lot more rainfall this year than we had last year.
She can speak English as fluently as her teacher (can).
I remember that we have learned this word before.
I don’t understand what you have said.
She asked the teacher how she could learn English well.
It depends on whether you want to do it or not
There is disagreement among economists about what money is and how money is measured.
Note: 在介词in后面跟that引导的宾语从句中，in that可看成是一个固定的搭配，表示“因为”或“在…方面”。
The evidence is invalid in that it was obtained through illegal means.
A turtle differs from all other reptiles in that it has its body encased in a protective shell of its own..