連詞分為並列連詞和從屬連詞兩大類，語法複習二十一：連 詞。並列連詞是連線主語與主語，謂語與謂語，句子與句子，分詞與分詞的詞，它要求前後兩部分有相同的形式和語法作用；而從屬連詞一般連線主句與從句，從句形式有名詞性從句，定語從句和狀語從句等。
（一）並列連詞：
並列連詞可用來連線詞與詞，片語與片語，分句與分句。常用的並列連詞有：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（因為），so（所以），while（而），when（這時）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（光榮的）yet difficult task./ Strike while iron is hot.
（二）從屬連詞
從屬連詞是指在複合句中引導從句的連結詞。常見的從屬連詞有：
引導時間狀語從句的：after, before, when, as, while, since, until, till, as soon as
引導原因狀語從句的：because, since, as
引導讓步狀語從句的：although, though, no matter（無論）, even if (though)
引導條件狀語從句的：if, unless, once, so (as) long as
引導結果狀語從句的：so … that …, such … that …
引導目的`狀語從句的：so that …, in order that …
引導比較狀語從句的：as … as …, not so (as) … as …, … than …
引導方式狀語從句的：as if …
引導主語，賓語或表語從句的連詞主要有：that, whether, if三個。其中that 和whether間或還可以引起同位從句和狀語從句。
（三）某些用法比較特殊的從屬連詞用法區別
1、當while, when, as引導時間狀語從句時的區別：①while引導的狀語從句中動詞必須是持續性。謂語動詞多為進行時，或狀態動詞的一般時。while 的這些用法可用when代替，等於 "at the time that", "during the time that"。例如：Please keep quiet while (when) others are studying；② when除可指一段時間外，還可用來指一點時間，等於 "at the time"，也就是說when引出的時間狀語從句中的謂語動詞可以是終止性的，也可以延續性的。因此主句和從句的謂語可以是一般時，進行時，或完成時。例如：When I went into the lab, the teacher was doing an experiment.（when不能換成while）He often makes mistakes when he is speaking English.（when可換成while）③as常可與when，while通用，但強調"一邊、一邊"。例如：As (when, while) I was walking down the street, I noticed a police car in front of number 37. ④when引導的狀語從句中的主語與主句主語一致，主、謂是"主語+系動詞"結構時，這時主語和系動詞可以省略。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有時代替if，引導條件句，意為"如果"、"假如"，例如：I'll come when (if) I'm free.
2、before作連詞一般表示時間，意為"在…之前"，但有些句子中這樣譯就顯得彆扭。試看以下句子的翻譯：He almost knocked me down before he saw me.他幾乎把我撞倒才看見我；Before I could get in a word he had measured me.我還沒來得及插話，他已經給我量好了尺寸。
3、till, until作為介詞式從屬連詞引導時間狀語短語或狀語從句，用於否定句時，結構為not …until (till)，主句謂語動詞延續與非延續皆可，意為"直到…才…"。用於肯定句時，只與延續性動詞連用，表示"到…為止"。例如：They played volleyball until (till) it got dark./ They didn't talk（延續性動詞）until (till) the interpreter（譯員）came./ He didn't go to bed（非延續性動詞）until (till) the his father came back.；until可以放在句首，till則不行，例如：Until the last minute of the match we kept on playing./ Not until he finished his work did he go home.（倒裝）；till, until只用於時間，以下句子是錯誤的：We walked till the edge of the forest.（要用as far as或to）。
4、because, since, as引導原因狀語時注意使用上的區別：①如果原因構成句子的最主要部分,一般用because ,因此because引導的從句往往放在句末。用why提問的句子，一定用because回答。例如：He had to stay at home yesterday because he was ill.；②如原因已為人們所知，或不如句子的其他部分重要，就用as，或 since，英語語法《語法複習二十一：連 詞》。since比as更正式些。as和since引導的從句一般放在句子的開頭。例如：As you are tired, you had better rest./ Since everyone is here, now let's begin.
5、although和though引導讓步狀語從句往往用法一樣，但注意以下區別：①although用於各種文體，而though則多用於非正式的口語或書面語中。注意由although, though引導的從句後，主句不能用but，但可用副詞yet, still。例如：Although/ Though it rained all the morning, they still went on working.（或yet they went on working）②though常與even連用，even though表示強調，意為"即使"，但不能說even although，例如：Even though I didn't understand a word, I dept smiling. ③though可用作副詞，意為"然而"，常用逗號與句子分開。although則不能這樣使用，它只作連詞。例如：It was a quiet party, I had a good time, though.
6、once作副詞譯"曾經"，作為連詞譯"一旦"，引導條件狀語從句。相當於if的加強形式。例如：I don't believe he was once a thief. (once這裡是副詞)/ 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連詞)
7、unless引導條件狀語從句等於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引導的方式狀語從句及表語從句中，根據情況要使用虛擬語氣。例如：He talks as if he knew all about it. 但有時也可用直陳語氣。例如：It looks as if it is going to rain.
9、whether, if引導從句的用法區別：①引導主語從句、表語從句或同位語從句時，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則不可。例如：I haven't decided whether to leave or not. ③whether可作介詞的賓語或置於句首表示強調，而if則不可。例如：Everything depends on whether we have enough money./ Whether he will come, I am not sure. ④whether和if均可引導賓語從句， whether引導的賓語從句一般都是肯定句，if引導的賓語從句可以是肯定的，也可以是否定的（此時不能用whether），例如：Could you tell us whether/ if it rains in winter in Australia?/ I wonder if it doesn't rain. ⑤引導賓語從句的whether和if常可與or not連用。連用時要注意or not的位置，它一般與 whether、if分開使用，有時它可與whether合起來使用，但不能與if合起來使用。例如：I don't know whether/ if they will come or not./ I don't know whether or not they will come. ⑥if可用來引導條件狀語從句，譯"如果"，whether則不行。例如：If you work hard, you are sure to succeed.
10、as作從屬連詞可引導多種狀語從句。①as引導時間狀語從句，意為"當…時"。例如：As (he was) a young man, he was a storekeeper and later a postmaster./ He sang as he worked. ②as引導方式狀語從句，意為"象…一樣"。例如：We must do as the Party teaches us. ③as引導原因狀語從句。意為"由於"，例如：As you are tired, you had better rest. ④as引導讓步狀語從句。意為"雖然"、"儘管"Child as he is, he can do it well. ( = Although he is a child, he can do it well.)另外，as做為關係代詞還可以引導定語從句，如：I have the same book as you.
練習、連詞
1 is very old,____ he still works very hard. A. but
2. ____ you are dismissed.
A. Neither you go nor B. Either you go or C. Whether you go or D. Both you go and
3. They had camped once before, ____ they knew what to take.
A. because B. now C. so D. since
4. Why these thi
