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.

练习、连词

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.

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.

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.)

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)

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.

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.

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)。

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。

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.