连词分为并列连词和从属连词两大类。并列连词是连接主语与主语，谓语与谓语，句子与句子，分词与分词的词，它要求前后两部分有相同的形式和语法作用；而从属连词一般连接主句与从句，从句形式有名词性从句，定语从句和状语从句等。
（一） 并列连词：
并列连词可用来连接词与词，词组与词组，分句与分句。常用的并列连词有：and（和），as well as（既…又），both … and（不但…而且），not only … but also（不但…而且），not … but（不是…而是），neither … nor（既不…也不），either … or（不是…就是），or（或者），but （但是），yet（然而），for（因为），so（所以），while（而），when（这时）等。如：Both my brother and my sister are teachers./ His room is bright but mine is gloomy（暗沉沉的）./ He can not only repair radios but also fix them./ It is a glorious（光荣的）yet difficult task./ Strike while iron is hot.
（二） 从属连词
从属连词是指在复合句中引导从句的连结词。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, so (as) long as
引导结果状语从句的：so … that …, such … that …
引导目的状语从句的：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 if …
引导主语，宾语或表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。
（三）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：
①while引导的状语从句中动词必须是持续性。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"。例如：Please keep quiet while (when) others are studying；
② when除可指一段时间外，还可用来指一点时间，等于 "at the time"，也就是说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。例如：When I went into the lab, the teacher was doing an experiment.（when不能换成while）He often makes mistakes when he is speaking English.（when可换成while）
③as常可与when，while通用，但强调"一边、一边"。例如：As (when, while) I was walking down the street, I noticed a police car in front of number 37.
④when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。例如：When (he was) young, he worked for a rich man./ She'll be here to give you help when (if it is) necessary.
⑤when有时代替if，引导条件句，意为"如果"、"假如"，例如：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 me.他几乎把我撞倒才看见我；Before I could get in a word he had measured me.我还没来得及插话，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"。例如：They played volleyball until (till) it got dark./ They didn't talk（延续性动词）until (till) the interpreter（译员）came./ He didn't go to bed（非延续性动词）until (till) the his father came back.；until可以放在句首，till则不行，例如：Until the last minute of the match we kept on playing./ Not until he finished his work did he go home.（倒装）；till, until只用于时间，以下句子是错误的：We walked till the edge of the forest.（要用as far as或to）。
4、because, since, as引导原因状语时注意使用上的区别：
①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。例如：He had to stay at home yesterday because he was ill.；
②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。例如：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：
①although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。例如：Although/ Though it rained all the morning, they still went on working.（或yet they went on working）
②though常与even连用，even though表示强调，意为"即使"，但不能说even although，例如：Even though I didn't understand a word, I dept smiling.
③though可用作副词，意为"然而"，常用逗号与句子分开。although则不能这样使用，它只作连词。例如：It was a quiet party, I had a good time, though. 6、once作副词译"曾经"，作为连词译"一旦"，引导条件状语从句。相当于if的加强形式。例如：I don't believe he was once a thief. (once这里是副词)/ Once Aristotle had made up his mind that heavy objects always fell faster than light objects, he taught it as a truth to his students. (once连词)
7、unless引导条件状语从句等于if … not …。例如：He'll accept the job unless the salary is too low. ( = He'll accept the job if the salary is not 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 it. 但有时也可用直陈语气。例如：It looks as if it is going to rain.
9、whether, if引导从句的用法区别：
①引导主语从句、表语从句或同位语从句时，用whether，不用if。例如：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.
②whether可接不定式，而if则不可。例如：I haven't decided whether to leave or not.
③whether可作介词的宾语或置于句首表示强调，而if则不可。例如：Everything depends on whether we have enough money./ Whether he will come, I am not sure.
④whether和 if均可引导宾语从句， whether引导的宾语从句一般都是肯定句，if引导的宾语从句可以是肯定的，也可以是否定的（此时不能用whether），例如：Could you tell us whether/ if it rains in winter in Australia?/ I wonder if it doesn't rain.
⑤引导宾语从句的whether和if常可与or not连用。连用时要注意or not的位置，它一般与 whether、if分开使用，有时它可与whether合起来使用，但不能与if合起来使用。例如：I don't know whether/ if they will come or not./ I don't know whether or not they will come.
⑥if可用来引导条件状语从句，译"如果"，whether则不行。例如：If you work hard, you are sure to succeed.
10、as作从属连词可引导多种状语从句。
①as引导时间状语从句，意为"当…时"。例如：As (he was) a young man, he was a storekeeper and later a postmaster./ He sang as he worked.
②as引导方式状语从句，意为"象…一样"。例如：We must do as the Party teaches us.
③as引导原因状语从句。意为"由于"，例如：As you are tired, you had better rest.
④as引导让步状语从句。意为"虽然"、"尽管"Child as he is, he can do it well. ( = Although he is a child, he can do it well.) 另外，as做为关系代词还可以引导定语从句，如：I have the same book as you.
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