連詞是一種虛詞, 它不能獨(dú)立擔(dān)任句子成分而只起連接詞與詞,短語與短語以及句與句的作用.連詞主要可分為兩類：并列連詞和從屬連詞.并列連詞用來連接平行的詞、詞組和分句.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. 并列連詞 并列連詞用來連接屬于同一層次并具有相同句法功能的詞；短語或句子.并列連詞包括：基本并列連詞如 and, or , but , 關(guān)聯(lián)連詞如 either...or , neither...nor , not only...but also ,both ...and, whether...or 等；介于并列連詞與從屬連詞之間的連詞；介于并列連詞與從屬連詞或介詞之間的結(jié)構(gòu)如 as well as , as much as, rather than , more than, no less than 等；此外還有些'半連接詞',一些語法學(xué)家把它們稱為連接副詞,如 nevertheless, however, meanwhile, otherwise, likewise 等,它們?cè)诰渲凶鲞B接性狀語.這類連詞主要是從邏輯上,而不是從形式上連接句子,其關(guān)系比較松散.
1) 表示意義轉(zhuǎn)折和對(duì)比的并列連詞
常見的有：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 等詞語.
2) 表示選擇的并列連詞
常見的有： or, whether...or, either...or, otherwise 等.例如：
Either ...or 和 whether...or 表示選擇,其意義比單用 or 要強(qiáng),但由 whetrher...or 構(gòu)成的并列結(jié)構(gòu)一般只能擔(dān)任句子的從屬成分.上例中最后一句, whether...or 結(jié)構(gòu)在句中擔(dān)任句子的從屬成分. Either ...or 和 or 一樣,可以連接兩個(gè)并列的獨(dú)立分句,而 whether...or 則不可以. or 用于表示否定的條件意義時(shí),有時(shí)可與連用.
Or 連接主語時(shí),如主語都是單數(shù),動(dòng)詞則取單數(shù)形式；如主語都是復(fù)數(shù),動(dòng)詞則取復(fù)數(shù)形式,如果主語有的是復(fù)數(shù),動(dòng)詞則取復(fù)數(shù)形式,如果主語有的是單數(shù),有的是復(fù)數(shù),動(dòng)詞的數(shù)則與它靠近的主語的數(shù)相一致.
連接兩個(gè)主語時(shí),動(dòng)詞的數(shù)也應(yīng)與靠近他的主語的數(shù)相一致.例如：
Neither he nor I am a good student.
3) 表示因果意義的并列并列連詞
常見的有： 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.
表示原因的并列連詞只有 for ,它所引導(dǎo)的分句只是對(duì)前一分句補(bǔ)充說明理由或推斷原因. for 引導(dǎo)的分句只能置于句末,而且必須用逗號(hào)與前一分句隔開.上面所提到的 so,therefore 等詞,有的語法學(xué)家把它們做為連接性狀語.
4) 表示聯(lián)合關(guān)系的并列連詞
常見的有： and, both...and, neither...nor, not only... but also 等.
當(dāng) neither...nor, not only ...but also 連接主語時(shí),動(dòng)詞的數(shù)則與靠近它的主語的數(shù)相一致. Both ...and 不能連接兩個(gè)以上的并列成分,也不能連接分句.例如： 誤：Both Mary swept the floor and Nancy mopped it.
5) 其它并列連詞
常見的有： as well as, more than, rather than, no less than 等.
(1) as well as 表示 '同' 和 '也' 的意義
as well as 用作并列連詞時(shí)它意義相當(dāng)于 not only...but also, 但側(cè)重點(diǎn)在后一并列成分上而 as well as 側(cè)重點(diǎn)卻在前一并列成分上 ,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 ... 一樣 ' 之意.
當(dāng) as well as ,more than, rather than, no less than 連接兩個(gè)成分作主語時(shí)謂語動(dòng)詞應(yīng)于第一個(gè)成分的數(shù)相一致. 在使用并列連詞時(shí)我們應(yīng)該注意:
(1) 并列連詞不可以連用.
(2) 有些連接性狀語副詞可以和某些從屬連詞對(duì)應(yīng)使用.
(3) 在 for 或 so that 引出的分句中如果主語與前一分句的主語所指相同其主語不可以省略. 同樣, 如果第二個(gè)分句是由連接副詞引出的其主語通常也不可省略. 例如:
2. 從屬連詞
從屬連詞用來引導(dǎo)名詞從句和各類的狀語從句.
從屬連詞按詞形分為簡(jiǎn)單從屬連詞,復(fù)合從屬連詞,關(guān)連從屬連詞.
1) 簡(jiǎn)單從屬連詞
常見的有：after, although, as, because, before, if, lest,once, since, that, till, unless, until, when, where, whether, while 等.例如：
2) 復(fù)合從屬連詞
由兩個(gè)或兩個(gè)以上單詞構(gòu)成的從屬連詞,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) 關(guān)聯(lián)從屬連詞 由兩個(gè)關(guān)聯(lián)構(gòu)成的,如： as...as, nore(less,-er)...than, no sooner...than, so ...as , so...that,such...sa , the...the, whether...or 等,
使用從屬連詞時(shí),應(yīng)該注意
(1) 由從屬連詞引導(dǎo)的狀語從句,其位置通常是可變的.
(2) 并列連詞之間之前不可以加其它連詞,而從屬連詞之前可以加并列連詞；連接副詞.
使用連詞時(shí), 還應(yīng)該注意:
1. because, for, since, as 的區(qū)別
because語氣強(qiáng), 表示客觀必然原因：例如: He is absent, because he is ill. 因?yàn)樯?所以他沒來.
比較: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 語氣輕,表示非客觀必然的原因,是主觀可改變結(jié)果的原因,甚至是猜測(cè)可能的原因:
He must be ill, for he is absent. “缺席”不一定是“生病”,只是交流猜測(cè).
for 不能放句首,它是并列連詞.
since, as 都是不講自明的原因, 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.