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.
70. I saw Jack yesterday. He told me that he would stay here _____ the end of this year.
A. at B. by C. for D. till
六. 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。
（一）并列连词：
并列连词可用来连接词与词，词组与词组，分句与分句。常用的并列连词有：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（因为），so（所以），while（而），when（这时）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（光荣的）yet difficult task./ Strike while iron is hot.
（二）从属连词
从属连词是指在复合句中引导从句的连结词。常见的从属连词有：
引导时间状语从句的：after, before, when, as, while, since, until, till, as soon as
引导原因状语从句的：because, since, as
引导让步状语从句的：although, though, no matter（无论）, even if (though)
引导条件状语从句的：if, unless, once, so (as) long as
引导结果状语从句的：so … that …, such … that …
引导目的状语从句的：so that …, in order that …
引导比较状语从句的：as … as …, not so (as) … as …, … than …
引导方式状语从句的：as if …
引导主语，宾语或表语从句的连词主要有：that, whether, if三个。其中that 和whether间或还可以引起同位从句和状语从句。
（三）某些用法比较特殊的从属连词用法区别
69. _____ defeated, they didn't lose heart.
A. In spite of B. Except for C. Though D. Until
68. ---May I attend your lecture, Mr. Green? ---Welcome _____ open arms.
A. with B. by C. in D. for
67. Some people say that we live _____ the age of computers.
A. in B. at C. with D. for
66. What idea can a man who is blind from birth have_____ color?
A. in B. / C. with D. of
65. Because of her devotion _____ music, she has become friendly with Mr. Zhang.
A. in B. to C. with D. on
64. One ____ five will have the chance to join in the game.
A. within B. among C. in D. from