连词是虚词，不能在句中单独担任成分，只起连接作用。按其用法，连词可分为两大类：并列连词和从属连词。
1.并列连词
连接具有并列关系的词、短语或句子。
⒈ 并列关系：and，not only…but also…，both…and…，neither…nor…
如：I used to live in Paris and London.
Both Jane and Jim are interested in fishing.
The weather here is neither too cold nor too hot.
She is not only kind but also honest.
⒉ 转折关系：but，yet，while（然而），when（然而，偏偏）
如：The car is very old but it runs very fast.
The problem was a little hard,yet I was able to work it out.
The winter in Beijing is very cold while that of Kunming is warm.
Why did you borrow the book when you had one?
⒊ 选择关系：or，not…but…，either…or…
如：Would you like to live or would you like to stay?
He is not a teacher but a writer.
You can come either on Saturday or on Sunday.
⒋ 因果关系：for
如：It must have rained last night,for the ground is wet now.
The leaves of the trees are falling. for already autumn.
5. 区别
① and和or
⑴ 并列结构中，or通常用于否定句，and 用于肯定句。
⑵ 但有时and 也可用于否定句。请注意其不同特点：
so还可与表示数量的形容词many，few，much，little连用，形成固定搭配。
⑴ so many 已成固定搭配，a lot of虽相当于many，但a lot of为名词性的，只能用such搭配。
⑵ so，that与such，that之间的转换即为so与such之间的转换。
2.从属连词
指在复合句中引导从句的连结词。
⒈ 常见的从属连词有：
引导时间状语从句的：after，before，when，as，while，since，until，till，as soon as
引导原因状语从句的：because，since，as
引导让步状语从句的：although，though，no matter（无论），even if (though)
引导条件状语从句的：if，unless，once，as (so) long as
引导结果状语从句的：so，so that，so…that…，such…that…
引导目的状语从句的：so，so that…，in order that…
引导比较状语从句的：as…as，not so (as)，as，than
引导方式状语从句的：as，as if…，as though
引导地点状语从句的：where，wherever
引导名词性从句（主语，宾语、表语或同位语从句）的连词主要有：that，whether，if三个。其中that和whether间或还可以引起同位从句和状语从句。
⒉ 某些用法比较特殊的从属连词用法区别
① 当while,when,as引导时间状语从句时的区别：
⑴ while引导的状语从句中动词必须是延续性。谓语动词多为进行时，或状态动词的一般时。while的这些用法可用when代替，等于“at the time that",“during the time that”
如：Please keep quiet while(when)others are studying.
⑵ when除可指一段时间外，还可用来指一点时间，等于“at the time；when引出的时间状语从句中的谓语动词可以是终止性的，也可以是延续性的。因此主句和从句的谓语可以是一般时，进行时，或完成时。
如：When I went into the lab,the teacher was doing an experiment.（不能用while)
He often makes mistakes when he is speaking English （then 可换成while)
⑶ as 常可与when，while通用，但强调“一边、一边”。
如：As(when/while) I was walking down the street, I noticed a police car in front of number 37.
⑷ when 引导的状语从句中的主语与主句主语一致，主、谓是“主语+系动词”结构时，这时主语和系动词可以省略。
如：When (he was)young. he worked for a rich man.
She'll be here to give you help when (if it is)necessary.
⑸ when有时代替if，引导条件句，意为“如果”“假如”。
如：I'll come when (if）I'm free.
② before作连词一般表示时间，意为“在.…之前”，但有些句子中这样译就显得别扭。
试看以下句子的翻译：He almost knocked me down before he saw.他且乎把我撞倒才看见我；
Before I could get in a word he had measure.没来得及插话，他已经给我量好了尺寸。
③ till,until作为介词式从属连词，只用于时间，引导时间状语短语或状语从句，用于否定句时，结构为not…until(till)主句谓语动词延续与非延续皆可，意为“直到……才……”。用于肯定句时，只与延续性动词连用，表示“到……为止”。
如：They played volleyball until(till)it got dark.
They didn't talk(延续）until(till) the interpreter（译员）came.
He didn't go to bed（非延续）until(till) the his father came back.
但是，until可以放在句首，till则不行。
如：Until the last minute of the match we kept>Not until he finished his work did he go home.（倒装）
④ because,since,as引导原因状语时注意使用上的区别：
⑴ 如果原因构成句子的最主要部分，一般用because 因此because引导的从句往往放在句末。用why提问的句子，一定用because 回答。
如：He had to stay at home yesterday because he was ill.
⑵ 如原因已为人们所知，或不如句子的其他部分重要，就用as，或 since。since比as更正式些。as和since引导的从句一般放在句子的开头。
如：As you are tired,you had better rest/Since everyone is here,now let's begin.
⑤ although和though 引导让步状语从句往往用法一样，但注意以下区别：
⑴ although 用于各种文体，而though则多用于非正式的口语或书面语中。注意由 although,though 引导的从句后，主句不能用but，但可用副词yet，still。
如：Although/Though it rained all the morming, they still went.
⑵ though 常与even连用，even though表示强调，意为“即使”，但不能说even although。
如：Even though he didn't understand a word,I dept smiling.
⑶ though可用作副词，意为“然而”，常用逗号与句子分开。lthough 则不能这样使用，它只作连词。
如：It was a quiet party,I had a good time though.
⑥once作副词译“曾经”，作为连词译“一旦”，引导条件状语从句。相当于f的加强形式。
如：I don't believe he was once a thief.（once这是副词）
Once Aristotle（亚里士多德） had made up his mind that heavy objects always fell faster than light objects,he taught as a truth to his students.(once连词）
⑦ unless引导条件状语从句等于if…not…
如：He'll accept the job unless the salary is too low.
⑧ 在用as if引导的方式状语从句及表语从句中，根据情况要使用虚拟语气。
例如：He talks as if he knew all about.但有时也可用直陈语气。
It looks as if it is going to rain.
⑨ whether，if引导从句的用法区别：
⑴ 引导主语从句、表语从句或同位语从句时，用whether，不用if。
如：whether they will go to the Great Wall is not known.
The question is whether we can finish the task on time.
The question whether we will take part in the physics contest has not been decided.
⑵ whether 可接不定式，而if则不可。
如：I haven't decided whether to leave or not.
⑶ whether 可作介词的宾语或置于句首表示强调，而if则不可。
如：Everything depends on whether we have enough money.
Whether he will come,I am not sure.
I was only listening to the radio with half an ear,while I was preparing some food.我正在做吃的东西，没太留心听收音机。
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