定语从句 that, which, who
只用that
1. 当先行词是下列不定代词或被它们修饰时much, little, none, all, few, every(thing), any(thing), no(thing) 等时，通常用that：
All that she lacked was training. 她缺的只是训练。
Have you everything that you need? 你需要的东西都有了吗?
The sleeping man’s subconscious mind retained everything that was said around him. 这位酣睡的人头脑的下意识能记住他周围的人说的话。
She would never do anything that was not approved of by her parents. 她父母不赞同的事她绝不会做。
2. 当先行词有the very, the only, the same 等修饰时，通常用that：
This is the only example that I know. 我知道的例子只有这一个。 Those are the very words that he used. 那是他的原话。
3. 当先行词有形容词最高级或序数词(包括last, next等)等修饰时，通常用that：
This is the best dictionary that I’ve ever used. 这是我用过的最好的词典。 The first thing that you should do is to work out a plan. 你应该做的第一件事是订个计划。
4. 当关系代词在定语从句中用作表语时，通常用that：
China is not the country (that) it was. 中国已不是过去的中国了。 5. 当先行词是一个既指人又指物的并列词组时，通常用that： They talked about the persons and things that most impressed them. 他们谈论了使他们印象最深的人和事。
6. 当要避免重复时：
Which is the course that we are to take? 我们选哪门课程？ 7．主句以there be 开头时
There is a seat in the corner that is still free. 角落里还有一个空着的位子
8．当主句是以who开头的特殊疑问句时，引导词只能用that。如： Who is woman that you talked with just now? 刚跟你谈话的那妇女是？9．the same as 与 the same that
the same as所引导定语从句的内容与先行词同类而不同物；the same that表示所引导的从句内容与先行词同类同物，即：同一事物。如：
She is wearing the same coat that I lost a few days ago. 她正穿着我几天前所丢失的上衣。(暗含：她偷了我的上衣)
She is wearing the same coat as I lost a few days ago. 她正穿着跟我几天前所丢失的上衣同样的上衣。(暗含：她所穿的上衣不是我所丢失的那一件)
只用which
1. 引导非限制性定语从句时：
The current, which is very rapid, makes the river dangerous. 水流湍急，使这条河很危险。
He has to work on Sundays, which he doesn’t like. 他得在星期天工作，他是不喜欢这样的。
The London team, which played so well last season, has done badly this season. 伦敦队上一个季度打得很好，这个季度却打得很差。
2. 直接放在介词后作宾语时：
She may be late, in which case we ought to wait for her. 她可能晚到，那样我们就要等等她。
The documents for which they were searching have been recovered. 他们找寻的文件已找到了。
This morning some port wine came, for which I have to thank you. 今天早上送来一些波尔图葡萄酒，为此我得向你道谢。
注：有时“介词+which”引导的定语从句可以转换成“介词+which+不定式”结构：
He had only the long nights in which he could study. =He had only the long nights in which to study. 他只有漫漫长夜可用来学习。
He had a couple of revolvers with which he could defend himself.= He had a couple of revolvers with which to defend himself. 他只有一两把手枪用来自卫。
3．当先行词本身是that时
What’s that which flashed in the sky? 只用who
用who (whom) 而不用that的情形
(1) 在非限制性定语从句中的先行词指人时，只能用who (whom)。如： My sister, who is studying abroad, sent me a beautiful present just before this Christmas. 我姐姐，她在国外学习，就在圣诞节前给我寄来了漂亮的礼物。
(2) 当定语从句对指人的先行词进行隔位修饰时，只能用who (whom)。如： Do you know the girl in front of the blackboard who is speaking to the students? 你认识黑板前面正在给学生们讲话的那位女孩子吗？
(3) 当先行词为people和those时，只能用who (whom)。如：
Those who want to go there come here please. 那些想去那儿的人请来这
(4) 当先行词为one, ones, anyone, everyone, none, all(指人) 时，只能用who (whom)。如：
Anyone who is against us is our enemy. 任何反对我们的人就是我们的敌(5) 先行词指人，而关系代词在从句中作主语时，一般多用who。如： The man who came here yesterday said he would come again in a few days. 昨天来这儿的那个人说过几天他会再来。
(6) 在there be句型中名词是人的定语从句多用who (whom)。如： There are students in our class who / whom you have met. 我们班有些学生你见过。
(7) 当句子中有两个指人的现行词分别带有两个定语从句时，若一个定语从句的引导词为that, 那么，另一个定语从句的引导词必定为who。如：
The boy that you met just now is Li Ming’s brother who just graduated from a university. 刚才你会到的那个男孩是李明的刚从大学毕业的弟弟。
when, as, while在引导时间状语从句时的主要用法及区别：
先看一道试题：
－I’m going to the post office.
－_____ you are there, can you buy me some stamps? A．As B．while C．Because D．If
根据本题语境，选项C、D显然不合题意，故答案应在A、B之间选择。为什么正确答案为B而不是A呢？
现在就看看when, as, while在引导时间状语从句时的主要用法及区别：
一． when表示“当……的时候”。从句中既可用延续性动词，又可用非延续性动词，这些动词既可以表示动作，又可表示状态。从句中的动作既可和主句的动作同时发生，又可在主句的动作之前或之后发生。如：
When I came into the room, I found him lying there asleep. Mary was having dinner when I saw her.
When you meet a word you don't know, consult the dictionary. She was beautiful when she was a girl.
二.如果when从句的主语与主句主语相同，谓语又是be动词，从句主语和be可以省略：当when从句主语与主句主语相同时，用when加分词可以替代状语从句。如：
When（you are）in trouble, ask her for help. When（I was）sleeping, I never heard a thing.
三.as表示\当……时\一面……一面\，\随着\。其具体用法如下：
1、表示\当……时\、\和……同时\。常指从句的动作未结束，主句中的动作就已发生。从句中多用表示动作的动词，而不用be动词或表示感觉、理解、知道这类动词。
As he stood there, he saw two men enter the bar. She dropped the glass as she stood up.
As I was walking down the street, an American asked me for directions to the nearest station.
2、用于平行的动作中，表示\一面……一面……\。常指一个主语同时进行两个动作。如：
He sang as he worked.
The students took notes as they listened.
3、用于平行发展的结构中，表示\随着……\。常指一个行为是另一个行为的结果，或一种状态随另一种状态变化。句中的动词多表示状态的发展变化。 As the wind rose the noise increased. As it grew darker it became colder.
As he grew older he became more confident.
四.While表示\在……的时候\、\在……期间\。它强调主句的动作与从句的动作同时发生或主句的动作发生在从句的动作过程中。While从句中必须是表示动作或状态的延续性动词。如：
Never get on or off a bus while it is in motion. The weather was fine while we were in Beijing. She called while I was out.
1、如果主句的动作在从句的动作过程中发生，从句常用进行时。如： While we were swimming someone stole our clothes. Don’t talk so loud while others are working.
2、如果主句和从句主语相同，while加现在分词可代替状语从句。如： We must strike while the iron is hot.
He took a bath while I was preparing dinner.
Come and pay a visit to the park while the flowers are in bloom. 三者的区别
通过以上分析我们可以看出，when, as, while引导时间状语从句时，侧重点有所不同。
1、When可指时间点，又可指时间段，从句中可用延续性动词，也可用非延续性动词。
2、While总是指一个时间段，从句中必须用延续性动词。 > 3、as多用于平行结构。表示两个动作同时发生：一个主语同时进行两个动作：一种状态随另一种状态的变化而变化。 4、表示主句的动作发生在从句的较长动作过程中，when, while, as均可使用。从句中为be动词时，多用when和while。谈论两个同时进行的长动作可用while；谈论两个平行动作或两种状态变化多用as。
再看所给的高考题，主句动词为buy，从句动词为be，不是两个平行动作，显然不应选项A，所以最佳答案为B。
词语辨析
1 表示“清楚”或“明显的”的词组obvious, clear，apparent，manifest，evident，distinct
Obvious ：easy to see or understand,常用于修饰或说明那些本想掩盖而又没能掩盖彻底的事情。
It’s obvious from what she said that something is wrong.根据她说的，显然是出问题了。 I know you don’t like her but try not make it so obvious .我明白你不喜欢她，但是尽量别表露的那么明显。
Clear :easy to understand and not causing any confusion是口语中的常见词，其含义是“清晰的，清楚易懂的”。此外，clear还有其他许多还难以，如“畅通的；清澈的；晴朗的；明亮的”等。例如：It was clear to everyone that the war would end quickly. 人人都明白战争将很快结束。
the clear water of the mountain lake 清澈的高山湖水 a clear sky 晴朗的天空 a clear photograph 清晰的照片 Apparent:
1,easy to see or understand;强调根据迹象进行仔细的推理，最后才看得出是“明显的” 2,seems to be real or true but may not be“表面上的”或“看上去似乎像的”
The effects of the drought are apparent to anyone who sees the parched fields. 任何一个看到龟裂田地的人，都明显的意识到是由于干旱造成的后果。 Manifest的含义是完全表现在外面，级所谓如此“明显的”，不需要推理便可辨别 There may be unrecognized cases of manifest injustice of which we are unaware. 也许有一些我们还不了解的未被承认的明显不公正的案例 Evident，总包含一定的迹象
It’s evident that the plan is feasible.很明显这计划是可行的。