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.

50. He is one of the old scientists who ______nothing about his personal income and fame .

　 A. cares　 　　　　　　　 B. care　 　　　　　　　　　　 C. is caring　 　　　 　　　 D. are caring

49. Nobody except Bill and Johnny ______entered the second round of the interview.

A. have　 　　　　　　　 B. has　 　　　　　　　　　　 C. have been　 　　　　　　 D. has been

48. Every hour and every minute ______ important.

　 A. are　　 　　　　　　　　　　 B. were　　 　　　　　　　　 C. is　 　　　　　　　　　　　 D. will

47.The pilot as well as four of the passengers ______death .

　 A. has escaped　　 　　 B. have escaped　 　　　　　　　 C. has been escaped　 　　 D. have been escaped

46. They each ______ a new dictionary.

　 A. has　 　　　　　　　　 B. have　　 　　　　　　　　 C. is　　 　　　　　　　　　　　 D. are

45. I have finished a large part of the book; the rest _______ more difficult.

　 A. is　 　　　　　　　　　　 B. are　 　　　　　　　　　　　　　　 C. was　　 　　　　　　　　　 D. were

44. I told him what I was surprised ______ his attitude towards his study.

　 A. is　　 　　　　　　　　　 B. was　　 　　　　　　　　　 C. at is　 　　　　　　　　　　 D. at was