此两个连词意义相同。肯定形式表示的意思是“做某事直至某时”，动词必须是延续性的。否定形式表达的意思是“直至某时才做某事”。动词为延续性或非延续性都可以。正确使用这两个连词的关键之一就在于判断句中的动词该用肯定式还是否定式。 肯定句：I slept until midnight. 我一直睡到半夜时醒了。Wait till I call you. 等着我叫你。(在肯定句中可用before代替。例如：Lets get in the wheat before the sun sets.) 否定句：She didn arrive until 6 oclock. 她直到6点才到。Don get off the bus until it has stopped. 公共汽车停稳后再下车。I didn manage to do it until you had explained how.直到你教我后，我才会做。 区别：1)until可用于句首，而till通常不用于句首。Until you told me, I had heard nothing of what happened.直到你告诉我以前，出了什么事我一点也不知道。 2)until when 疑问句中，until要放在句首。Until when are you staying?Until next Monday.你呆到什么时候?呆到下周一。注意：否定句可用另外两种句式表示。 3)Not until…在句首，主句用倒装。Not until the early years of the 19th century did man know what heat is.直到19 世纪初，人类才知道热为何物。Not until I began to work did I realize how much time I had wasted.直到工作，才认识到我已蹉跎了许多岁月。 4)It is not until… that….It was not until I began to work that I realized how much time I had wasted. 巧辨before和until 在日常英语教学中，我们遇到学生问这样的一些问题：1.He will spend six hours at his desk____he finishes his composition.A.before B.until C.after D.when 2.The bike hit the tree____I could get off.A.when B.before C.while D.until 3.I knocked at the door for more than five minutes____Mrs white answered it.A.until B.when C.after D.before 4.It was____yesterday____be____the secret.A.not until;that;knew B.until;when;knewC.not before;that;found D.before;that;didn find 怎样才能清楚地解释其选择的原因，这就涉及到until与before的区别问题。实际上，只要我们把握住两者使用时本身的含义及主句动词是终止性的，还是延续性的，肯定式，还是否定式两大点，就能容易地解决这类问题。 现详细阐明如下：一、在下列情况下，两者可互换用，但含义略有不同。before表示“在……之前”的意思，强 调时间先后关系;而until表示“直到……才”的意思，主句是肯定句则表示主句动作的终止时间;主句是否定句则强调主句动作的起始时间。1.主句为否定式终止性谓语动词。常见的动词open,start,leave,arrive,finish, stop等。(1)The noise of the street didn stop until/before it was midnight.(2)The children won come back until/before it is dark.(3)I didn leave the lovely boy until/before his mother came home.2.主句为肯定式、延续性谓语词，这类动词用stand,stay,talk,be,wait等。(1)He lived with his parents until/before he graduated from school.(2)I will wait until/before he comes to my help.(3)I shall stay heer until/before you come back. 二、在下列情况下，用before不用until。1.主句这肯定式、终止性谓语动词，只用before。(1)He fell asleep before I could take off his clothes.(2)We arrived there before it atarted to rain.(3)The holiday came to the end befor I knew it.(4)He almost knocked me down before he saw me.2.主句谓语动词强调动词的迟缓性，只用before，常伴有时间段状语或时间段暗示。(1)It was quite some time before he found the elephant at all.(2)We had walked a long way before we found some water.(3)We had sailed for two days before we saw the land.(4)The fire lasted about four hours before the firefighters could countrol it.(5)It was midnight before my brother came home.3.如果强调从句谓语动作未发生，就发生主句谓语动作，只用before。常译为“未及”，“不”或“不等……就”。(1)We can leave early in the morning before it gets too hot.(2)I must write it down before I forget it.(3)We do want to buy something now before prices go up.4.表“与其说……倒不如”，“与其……毋宁……”只用before。(1)I would give up my job before Id agree to be dismissed.(2)He will die of hunger before he will steal.5.在某些特定句型中用before。(1)It was not long before the whole country rose up.(2)It will probably not be long before they understand each other. 三、下列情况一般区别用until1.主句是持续性谓语动词时，肯定、否定都可以，但意义完全不同。肯定表动作终止，而否定表动作开始。(1)We discussed the problem until/before he came back.我们一直讨论到他回来。(2)We didn discuss the problem until he came back.我们一直等到他回来后才讨论问题。2.not…until句型尽管在某些情况下可与before互换用，但在强调句中一般仍用until。(1)It was not until he told me that I knew it.(2)It was not until he finished his homework that he went home.综上所述，前面的4条选择题中，第1、3题强调动作发生的迟缓性应选before。第2题表动作未及发生就发生主句动作也应选before。第4题为强调句应选A项。 其他时间状语从句的用法 1.由since引导的时间状语从句。since引导的从句的谓语动词可以是延续性的动词，又可以是瞬时动词。一般情况下，从句谓语动词用一般过去时，而主句的谓语动词用现在完成时。但在It is +时间+since从句的句型中，主句多用一般现在时。I have been in Beijing since you left. 自从你离开以来，我一直在北京了。Where have you been since I last saw you?自上次我和你见面以后，你到哪里去了?It is four years since my sister lived in Beijing. 我妹妹不在北京住有四年了。知识扩展It is since从......以来多长时间了(因为since +从句或名词，表示一段时间)It is five years since we met last time.从我们上次见面已经五年了。It is +before(......才)It was a long time before I went to sleep again. 过了很长时间我才睡着。It was an hour before(=until) the police arrived. 过了一个小时，警察才来。 2.由as soon as, immediately, directly, instantly, the moment, the instant, the minute, 等引导的时间状语从句，这些连词都表示“一......就”。I will go there directly I have finished my breakfast.吃完早饭，我立即到那里去。The moment I heard the news, I hastened to the spot.我一听到消息，马上赶到了出事地点。As soon as I reach Canada, I will ring you up.我一到加拿大，就给你来电话。 3.由by the time引导的时间状语从句。注意时态的变化：在一般情况下，如果从句的谓语动词用一般过去时，主句的谓语动词用过去完成时;如果主句的谓语动词用一般现在时，主句的谓语动词用将来完成时。By the time you came back, I had finished this book.到你回来时，我已经写完这本书了。By the time you come here tomorrow, I will have finished this work.你明天来这儿的时候，我将已经完成此工作了。 时态问题 在状语从句中，有“主将从现”的规定，即主句用一般将来时，从句用一般现在时表示将来时。 注意：when 、while 、as 、after、 before、 as soon as、 since、 till /until 、by the time 在时间状语从句中，要注意时态一致。一般情况下主句是将来时的时候，从句要用一般现在时。I will call you as soon as I arrive in Beijing. 我到北京就将给你打电话。(这是由as soon as引导的时间状语从句，从句中的谓语动词arrive是一般现在时，表示一般将来时，决不可用will arrive)
