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)

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

　　　　　　　　　　　　　　　　 　连 词 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

17. When the last prize had been awarded _____ everybody cleared off.

　A. and　　　　　　　 B. so　　　　　 　　　C. or　　　　　D. 不填

16.. Although he had only entered the contest for fun, _____ he won first prize.

　A. but　　　　　　　　B. andw　　　　　　　 C. even　　　　D. 不填

15. Just because they make more money than I do, _____ they seem to look down on me.

　A. so　　　　　　　　B. and　　　　　　　 C. but　　　　D. 不填

14. The point is not who said the words, _____ they are true or not.

　A. but whether　　　B. and whether　　　 C. but how　　D. and how

13. I’m sorry, _____ I won’t be able to come tonight.

　A. for.　　　　　　 B. and　 　　　　　　C. but.　　　 D. then

12. _________ the fact that he was not a trained economist, his achievements were remarkable.

A．In spite of　　 　 　B．In case of　　　　 C．In view　　 D．In favor of

11. --- Would you please tell me what time it is?　

　　 --- It’s five minutes to nine_______my watch.

A．on　　 　　　　　　 B．by　 　　　　　　 C．at　　 　　　D．in