连词是一种虚词, 它不能独立担任句子成分而只起连接词与词,短语与短语以及句与句的作用.连词主要可分为两类：并列连词和从属连词.并列连词用来连接平行的词、词组和分句.如：and, but, or, nor, so, therefore, yet, however, for, hence, as well as, both…and, not only…but also, either…or, neither…nor, (and)then等等.
1. 并列连词 并列连词用来连接属于同一层次并具有相同句法功能的词；短语或句子.并列连词包括：基本并列连词如 and, or , but , 关联连词如 either...or , neither...nor , not only...but also ,both ...and, whether...or 等；介于并列连词与从属连词之间的连词；介于并列连词与从属连词或介词之间的结构如 as well as , as much as, rather than , more than, no less than 等；此外还有些'半连接词',一些语法学家把它们称为连接副词,如 nevertheless, however, meanwhile, otherwise, likewise 等,它们在句中做连接性状语.这类连词主要是从逻辑上,而不是从形式上连接句子,其关系比较松散.
1) 表示意义转折和对比的并列连词
常见的有：but , while, whereas, still, yet , nevertheless, likewise, anyway , only , conversely , on the contrary, by this time, all the same , fortunately, on the other hand , in the meantime 等词语.
2) 表示选择的并列连词
常见的有： or, whether...or, either...or, otherwise 等.例如：
Either ...or 和 whether...or 表示选择,其意义比单用 or 要强,但由 whetrher...or 构成的并列结构一般只能担任句子的从属成分.上例中最后一句, whether...or 结构在句中担任句子的从属成分. Either ...or 和 or 一样,可以连接两个并列的独立分句,而 whether...or 则不可以. or 用于表示否定的条件意义时,有时可与连用.
Or 连接主语时,如主语都是单数,动词则取单数形式；如主语都是复数,动词则取复数形式,如果主语有的是复数,动词则取复数形式,如果主语有的是单数,有的是复数,动词的数则与它靠近的主语的数相一致.
连接两个主语时,动词的数也应与靠近他的主语的数相一致.例如：
Neither he nor I am a good student.
3) 表示因果意义的并列并列连词
常见的有： for ,so ,therfore ,hence ,thus, accordingly, consepuently, on that account, in that case 等.例如：
The fuel must have beeenfinished, for the engine stopped.
It rained , therefore the game was called off.
表示原因的并列连词只有 for ,它所引导的分句只是对前一分句补充说明理由或推断原因. for 引导的分句只能置于句末,而且必须用逗号与前一分句隔开.上面所提到的 so,therefore 等词,有的语法学家把它们做为连接性状语.
4) 表示联合关系的并列连词
常见的有： and, both...and, neither...nor, not only... but also 等.
当 neither...nor, not only ...but also 连接主语时,动词的数则与靠近它的主语的数相一致. Both ...and 不能连接两个以上的并列成分,也不能连接分句.例如： 误：Both Mary swept the floor and Nancy mopped it.
5) 其它并列连词
常见的有： as well as, more than, rather than, no less than 等.
(1) as well as 表示 '同' 和 '也' 的意义
as well as 用作并列连词时它意义相当于 not only...but also, 但侧重点在后一并列成分上而 as well as 侧重点却在前一并列成分上 ,A as well as B=not only B but also A.
(2) more than 表示而不是之意. 例如:
(3) rather than 表示 '而不是' 之意.
(4) no less than 表示 ' 同 ... 一样 ' 之意.
当 as well as ,more than, rather than, no less than 连接两个成分作主语时谓语动词应于第一个成分的数相一致. 在使用并列连词时我们应该注意:
(1) 并列连词不可以连用.
(2) 有些连接性状语副词可以和某些从属连词对应使用.
(3) 在 for 或 so that 引出的分句中如果主语与前一分句的主语所指相同其主语不可以省略. 同样, 如果第二个分句是由连接副词引出的其主语通常也不可省略. 例如:
2. 从属连词
从属连词用来引导名词从句和各类的状语从句.
从属连词按词形分为简单从属连词,复合从属连词,关连从属连词.
1) 简单从属连词
常见的有：after, although, as, because, before, if, lest,once, since, that, till, unless, until, when, where, whether, while 等.例如：
2) 复合从属连词
由两个或两个以上单词构成的从属连词,如： as if, as far as , as soon as, according as , in case , no matter who(how,what,when, where), rather than, for all that , given( that), in order that, now (that), on condition that , (so) that ,provided/providing(that), inasmuch as , insofar as 等.
3) 关联从属连词 由两个关联构成的,如： as...as, nore(less,-er)...than, no sooner...than, so ...as , so...that,such...sa , the...the, whether...or 等,
使用从属连词时,应该注意
(1) 由从属连词引导的状语从句,其位置通常是可变的.
(2) 并列连词之间之前不可以加其它连词,而从属连词之前可以加并列连词；连接副词.
使用连词时, 还应该注意:
1. because, for, since, as 的区别
because语气强, 表示客观必然原因：例如: He is absent, because he is ill. 因为生病,所以他没来.
比较:He is absent, for he is busy. （“生病”是“缺席”的必然原因,“忙”不是必然原因.）
for 语气轻,表示非客观必然的原因,是主观可改变结果的原因,甚至是猜测可能的原因:
He must be ill, for he is absent. “缺席”不一定是“生病”,只是交流猜测.
for 不能放句首,它是并列连词.
since, as 都是不讲自明的原因, 是已知的原因.
Since I am a boy, let me carry the case.
As you don't feel well,you had better stay at home.
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