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before和ago都能表示在一段时间以前，但是二者的内涵和具体用法有明显的区别：1. before表示从过去或将来某一时间算起多少时间以前，由于与这一时间状语连用的行为先发生在过去或将来某一时刻之前，因此往往出现在过去完成时态或将来完成时态中。
例如：①I called him up last night，but his mother told me he had left for Wuhan two days before.②She said that she had married him five years before.2.如果不具体表明多少时间以前，before意为"从前；以前"，表示从现在开始追溯到过去一段时间，或者主句的谓语动词表示的是经常、反复发生的行为，以副词的形式置于句末，可以用在现在完成时态中。
例如：①I have never been to Beijing before.②Have you ever seen a tiger before?③I often go over the lessons that I have learned a few days before.注意：before前的时间状语如果是特定的，含义不一样。
试比较：He said he had arrived here a day before. (一天前)He said he had arrived here the day before. (前一天)3. ago指的是从现在起到过去时间的某一点的一段时间。
这个时间状语常与一般过去时态或过去进行时态连用。
例如：①I graduated from the college twenty years ago.②I was asked what I was doing a month ago.4. 表示在某一时间点或事件以前时，只能用before，不用ago。
介词的种类及用法（二）一，表示时间的介词（in、with；before、after；from、since）1. in、with1). in + 一段时间……后；（此用法多用于一般将来时和过去时）in an hour一小时以后；in five days五天后；in twenty minutes 二十分钟后①.It would strike twelve in twenty minutes' time.再过20分钟，大钟将敲响12下。
②.We will reach the port in three days.我们将在三天后抵达港口。
2).within 在……之内within three days 三天内☞He finished a thick novel within three days.他在三天内就看完了一本厚厚的小说。
●●●辨析：within 和inwithin 强调“在……时间之内”，没有时态的限制in 指从现在起到以后一段时间，只用于一般将来时2. before、after1). before在…以前；☞I am going to finish my homework before Sunday night.我打算在周日晚上之前把作业做完。
2). after在…之后after class 下课后；after school 放学后；after lunch 午饭后☞After school, she plays volleyball with her students.放学后，她与她的学生们一起打排球。
3. from、since1). from 从……起；from…to…从…到；（也表示空间的概念或数字的范围）☞From Monday to Friday are the weekdays.从星期一到星期五是工作日。
2). since 自从……以来（从过去某一时间一直到现在仍在持续）since+ 点时间;①.haven't seen Tom since last year.我从去年就没有见过汤姆。
since、for时间和by、before时间有什么不同？.私信小编，回复数字18，可以得到10G的英语学习资料和18大语语音技巧。
表时间的介词（下）1since和for这两个介词引导的时间状语都有延续的含义，但是着眼点不同。
“for+时间段”表示“延续….之久”，其中：与现在完成时连用，表达从过去延续到现在结束；与过去时连用，表达过去一段已经终结的时间；与将来时连用，表示延续到将来的一段时间。
例句：Some even have been planting this dream in their mind for years.有些人甚至多年来一直在他们的脑海中有这样的憧憬。
（完成时）I organized an English club and was the captain of the club for a year.我组织过一个英语俱乐部，并担任了一年的队长。
（和过去时）They will stay in China for two years.他们将在中国待上两年。
（和将来时）“since+时间点”，表达从那个点开始，延续到现在，所以翻译为“自从（过去某个时间）以来”，既然跨度为两个时间，自然要用完成时。
例句：They have been friends since childhood.他们从幼时起一直是好朋友。
She's been skiing since childhood.她从幼时起就开始滑雪。
It's been an age since we met.我们好久未见面了。
2by和beforeBy后面加时间点，表达“不晚于某时”，也可以翻译为“截止到”。
例如：By the 1920s, he had become an explorer, searching for the tombs of the Egyptian kings.到20世纪20年代的时候他已经成为寻找古埃及王墓方面的探险家。
时间状语从句before 和since的用法before 和since的用法1.连词before表示“还未...就...；不到...就...；...才...；趁...还没...”Before they reached the station, the train had gone.他们(还没)到达火车站前，火车已经开走了。
We hadn’t run a mile before he felt tired.我们跑了还不到一英里他就累了。
Please write it down before you forget it.趁你现在没忘把它记下来。
2.before的句型：It will be +时间段+before从句，表示“多久之后才”。
It won't be long +before从句，表示“不久之后将...”It will be half a year before I come back.半年后我才回来。
(一般现在时表将来)It won’t be long before we meet again.不久后我们将再见面。
3.since的句型：It is/has been+时间段+since从句。
若从句的谓语为非延续性动词，表示“自从动作开始以来有多久了”；若从句的谓语为延续性动词，表示“自从动作结束/完成...有多久了”，常译为“自从不...有多久了”。
It is three years since the war broke out.战争爆发以来，有三年了。
It is twenty years since I've seen her. 我已经二十年没见她了。
五、every time, each time, next time, the first time, any time, all the time 的用法这些短语用来引导时间状语从句，表示“每当..；每次...；下次...”等。
It is +时间段+ since从句和It was / will be + 一段时间 + before从句1.表示过去的情况在“It is +时间段+ since从句(从句用一般过去时)”句型中，如果since从句的谓语动词是瞬间性动词，表示肯定意义；如果since从句的谓语动词是延续性动词，则表示否定意义。
1) It is / has been ten years since he left Beijing. 自从他离开北京已经十年了。
分析：since从句中的left是leave的过去式，是瞬间性动词，表示肯定的意思，left译为“离开”2) It is / has been ten years since he was a doctor. 他不当医生已经十年了。
分析：since从句中的系表结构表示一种状态，这种状态是延续性的，表示否定意义，译为“不是，不当”注：时间的计算一律从since从句的动作完成或状态结束时算起。
2.表示过去的情况“It was+ 一段时间 + since从句(从句用过去完成时had done)”，主句用一般过去时，从句用过去完成时；“It had been + 一段时间 + since从句(从句用一般过去时)”即主句用过去完成时，从句用一般过去时。
It was three years since Jack had been a teacher. 杰克不当老师已经三年了。
3. It was / will be + 一段时间 + before从句1) 在“It was + 一段时间 + before从句”句型中，若主句是一般过去式，从句也是一般过去式时：①若主句是肯定句，翻译成“过了多长时间之后才……”It was long before they found their son. 过了很久他们才找到他们的儿子。
②若主句是否定形式，翻译成“没过多久就……”It was not long before the couple met again. 没过多久这对夫妻就见面了。
after，before，since用法区别的用法小结1. 用作介词，与一段时间连用表示“在…之后”时，注意它与in的区别：after表示以过去时间为起点的一段时间以后，且通常与过去时态连用；而in则表示以现在为起点的一段时间以后：He came backafter two days. 他两天后就回来了。
He will come back in twodays. 他过两天就回来。
但after 若是与一点时间连用，则可与过去时态也可与将来时态连用：He came back after 4 o’clock. 他是四点钟以后回来的。
He will come back after next month. 他将在下个月以后回来。
2. 用作连词引导时间状语从句时注意：若主句用将来时态(或含有将来意味)，则时间状语从句要用一般现在时态(表将来意义)，而不能直接使用将来时态：他回来后我就走。
正：I’ll leave after he comes back.误：I’ll leave after he will come back.3. 用作副词，表示“后来”、“ (一段时间)以后”，通常放在表示时间的副词之后：soon after 不久以后two days after 两天后4. 用于习语after all 终究，毕竟：I was right after all. 毕竟我是对的。
After all, he’s only six years old. 毕竟，他才六岁。
的用法小结高三课本第十五单元，INTER GATING SKILLS 和WORKBOOK READING部分有如下的句子：⒈The shop was successful，but it was about sixty years before people started to buy jeans for fashion as well as for work.⒉However，it probably will not be long before the other mountainous areas of China such as Sichuan，Xinjiang and Yunnan are developed，and as more facilities become available，prices will almost certainly fall.其中涉及到before的用法，下面就其用法进行总结。
高考中before和since的考点和用法Before:(1) 假设主句是一般时将来时态，从句是一般现在时.假设主句是肯定句意思是“要过多长时间才〞；假设主句是否认形式翻译成“用不了多久就〞。
It will be two weeks before everything returns to normal.两周之后一切才能恢复正常。
It won‘t be long before we meet again.用不了多久我们就会再见面的。
(2)这时假设主句是一般过去时从句也是一般过去时。
假设主句是肯定句翻译成“多长时间之后才〞，假设主句是否认形式翻译成“没过多久就〞。
如：It was some time before I realized the truth.___________________________________________。
It wasn‘t long before she became a brave solider.____________________________________________。
①Several weeks had gone by ________ realized the painting was missing.A.asB.beforeC.sinceD.when②--How long do you think it will be ________ china sends a manned spaceship to the moon？--Perhaps two or three years.A .when B.until C.that D.beforeSince:⑴第一种是从句用现在完成时，从句用一般过去时，表过去某个时间开始直到现在．如：〔1〕I have not laughed since I became deaf . 自从我聋了以后我就没有大笑过．〔2〕It's been years since I enjoyed so much as last night . 我已经好多年没有像昨晚那样痛快了．⑵第二种是主句用过去完成时，从句用一般过去时或过去完成时，表过去某个时间知道过去某个时间．如：Since she came to me , we had been happy . 自从她来到我这里，我们一直很快活．Two weeks had passed since Martin had seen him . 自从马丁见到他以后，两周已经过去了．4. He bought the TV set two years ago.It is two years _________ he ___________ the TV set. (同义句转换)5. Susan has been in this city for more than ten years. (同义句转换)______ more than ten years _____ Susan _________ to this city.1. We haven’t _______each other for a long time since we ______ last time.A．saw, met B. seen, have met C. seen, met D. seen , has met2. The old people _____ lonely at all since we ______ to visit them once a week.A. don’t feel, beginB. hasn’t felt , beganC. haven’t felt , beganD. didn’t feel , begun3. It is three years since he _________ his hometown.A. leavesB. leftC. is leavingD. has left3. The America Civil War lasted four years ______ the North won in the end.A.afterB.beforeC.whenD.then4. It was some time _______ we realized the truth.A.whenB.untilC.sinceD.before5. -Why didn‘t you tell him about the mee ting？-He rushed out of the classroom _______ I could say a word.A.beforeB.untilC.whenD.after。
since的用法主要有两点：一是表示“自从……以来”，二是表示“既然”之意。
但如果只了解词的含义，而不了解其用法，在使用时就会造成错误。
例如：I have not heard from him since I lived in Shanghai.不少人会望文生义，将该句译为：“自从我住在上海以来，就再也没有收到他的信了。
”而实际上这个句子的译文并非如此，那么正确的含义是什么呢？下面谈谈since作连词引导时间状语从句的用法。
一、若since引导的状语从句的谓语动词是终止性的过去时，则从句表示的时间是“从动作开始的那一时刻起”。
如：He has studied very hard since he came to our school.自从他来到我们学校，他学习就非常努力。
We have been missing them since they left here自从他们离开这里，我们就一直很想念他们。
这是最常见的一种用法，此处不再多述。
二、若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去时时，则从句表示的时间是“从那持续动作或状态结束时算起”。
如：I haven’t heard any noise since I slept. Sleep 为持续性动词，sleep的动作结束时，即“醒来”时，这句应译为“我醒后还未听到任何声音”。
John is now with his parents in New York, it is already three years since he was a teacher约翰现在和父母一起住在纽约，他不当教师已经三年了。
本文开始出现的那个句子就可以译为“自从我离开上海，就再也没有收到他的信。
”这种译法常被人们误用，我们不妨再多举几个例子看看：How long is it since you lived in Shanghai?你离开（没住在）上海多久了？It is half a month since he was a League member.他退团（不当团员）半个月了。
It+be+……+that/since/before/when用法辨析1.it was+ (时刻，或时间点）+ when sth. happened。
当某事发生时,是几点钟或哪一年如：It was 1997 when Hongkong returned to China。
It was five o’clock when we arrived at the small mountain village。
对比强调句型：It was at five o’clock that we arrived at the small mountain village。
It+ is/ was+…+that…多用于强调。
判断方法如下：去掉it、be和that，余下的部分仍是一个完整的句子,这样的句子一定是强调句。
2.it was+一段时间+ before sth happened。
过了一段时间之后,某事才发生了.如：it was two years before Macao returned to China.it will be +一段时间before sth。
happens.要过一段时间之后,某事才会发生。
或不用过多久，某些就将发生：it won't be long before sth。
happens。
It won’t be long before you see him again。
没过多久就…It was long before they escaped from the prison。
过了很久才….3。
it is (=has been)+一段时间since sth。
happened。
自某事发生以来,（时间）已经过去.。
了。
如：It is 11 years since Hongkong returned to China.香港回归已经有11年了。
若从句中的动词是终止性动词，意为：“从开始……多久了”例如：It has been five years since she left home.(她离家已经十五年了）It was four years since he had died.（到当时为止，他已死去四年了）若从句中的动词是延续性动词，意为：“不做某事已有多久了”例如：It is two days since she was ill.（她病好了两天)It is two years since he studied English。
since和before的用法时态
"Since" 和 "before" 都可以用于表示时间或事件的关系，但它们在句子中所表示的时间关系略有不同。
1. "Since" 在句子中表示自从某个时间点或事件之后一直持续到现在。
通常用于现在完成时或过去完成时等时间时态。
例子：
I have lived in this city since 2010. 我自2010年以来一直住在这个城市。
She had been working in the company since she graduated from college. 她自从大学毕业后一直在这家公司工作。
2. "Before" 在句子中表示在某个特定时间点或事件之前发生的动作。
通常用于过去时或过去完成时等时间时态。
例子：
He left the house before I arrived. 他在我到达之前离开了房子。
She had finished her homework before her parents came back. 她在父母回来之前已经完成了作业。
