时间状语从句
  用句子表达一件事情或一个行为发生的时间，这个句子就叫时间状语从句。时间状语从句根据其所表示的时间不同，所使用的连词也不同。下面我们一起来看一下几个常见的时间状语从句：
1．when引导的从句：表示主句的动作和从句的动作同时发生，或从句的动作发生在主句动作之前。
例如：I was going out when a visitor came. (动词动作同时发生。)我正要出门，有客人来访。
I shall tell her the good news when she comes.她来时，我将告诉她这个好消息。
小试牛刀：I found mother cooking for me. I got home.(when)_______________________________________
He was listening to the radio when I                    home. (get)
2．while 引导的从句：表示“当……过程中”，强调某一段时间内主句和从句中谓语动词所表
示的动作在同一时期发生。由于while从句表示一个持续的行为，所以从句中要用持续性动词，并且持续性动词常用进行时态。
例如：Keep an eye on my little sister while I am away.我不在时，请照看一下我的小妹妹。
While we were watching TV, Jim came in. 我们看电视的时候，Jim进来了。
小试牛刀：It started to rain. We were playing football.(while)_______________________________________
Lucy was cleaning the room         Lily was listening to music.
3．as引导的时间状语从句：表示“当……的时候；一边……一边……；随着……”，其从句的主语与主句的主语往往是同一人或物。
例如：She sang as she worked. 她一边工作一边唱歌。=She was singing as she was working.
I saw him as he was getting off the bus.  正当他下公共汽车时, 我看见了他。
小试牛刀：               we walked, we talked.
You will grow wiser           you grow older.
4．before引导的从句：一般表示主句的动作发生在从句动作之前。
例如：I didn’t know any English before I came here.  我到这儿来之前，一点英语都不懂。
例如：I had written my report before my father came back.
小试牛刀：He bought the ticket         he went to see the film.
5．after引导的从句：表示主句的动作发生在从句动作之后。
例如：After he locked the door, he left. 他锁上门后，就走了。
After he had finished his work, he played a game of chess with his friend. 他做完工作后，和朋友下了一盘棋。
小试牛刀：I will go out to play basketball with you. I finish my homework.(after)
        _______________________                                  _
          He left the classroom           he had finished his homework the other day.
6．since引导的从句：表示“自从……以来”，从句中的动词一般表示动作的起点，用过去时。主句中的动词表示动作延续的情况，一般用现在完成时。如果主句中的动词表示的不是延续的动作而是表示目前的状态，可用一般现在时。
例如：Since he met this good friend, he has made great progress in his studies.
自从遇见了这个好朋友，他已在学习上取得了巨大的进步。
例如：It is twenty years since she joined the Party. 她入党二十年了。（主句中的动词表示的是状态，用现在时。）
小试牛刀：it is five years since we             last time.（meet）
        Mr green has taught in that school since he             (come)  to China three years ago.
7．until 引导的从句：表示一个动作持续到某一时刻。
（1）表示“直到……”，如果主句中的谓语动词是延续性动词时，用肯定或否定形式都可以，但表达的意思不同。
（2）表示“直到……才”，主句用否定式，从句用肯定。如果主句中的谓语动词是瞬间动词时，必须用否定形式。
例如：They worked until it was late.  他们一直工作到天黑。
She didn’t go to bed until she finished her homework.  她写完作业才睡觉。
小试牛刀：All the people left. He waited.(until)________________________________________
He went to bed . his father came home.________________________________________
8．as soon as 引导的从句表示“一……就”，强调动作紧密相连。
例如：As soon as she came to the classroom, she began to read English.她一到教室，就开始读英语。
小试牛刀：We start to sing. It''s 12 o''clock. (as soon as)________________________________________begin和start区别
9. by the time 到。。。为止 (所在句子的主句应用完成时)
例如：By the time he gets there , his father has already gone.他到家的时候，他爸爸已经走了。
小试牛刀：By the time I got to school, the class                   .（begin）我到校时，已经开始上课了。
知识扩展
1. it is +时间段+since+一般过去时（从。。。以来多长时间了）
例如：it is five years since we met last time.从我们上次见面已经五年了。
     
2. it is/ will be +时间段+before+一般现在时  ， it was+时间段 +before+一般过去时  (。。。才)
  例如：it is a long time before I go to sleep .　过了很长时间我才睡着。
        it was an hour before the police arrived.过了一个小时，警察才来。
小试牛刀：It_____(be) ten years since we ________(leave) that city . 
          It is reported that it may be two or three months_____it is possible to test this medicine on animals
          How long do you think it will be ______ China sends a manned spaceship to the moon？
3、when和while的区别
连词
区别
所接动词
例句
when
主从句同时发生、
先后发生
瞬间性动词、持续性动词
When you arrive in London, please give us a call.
When he was having his lunch, he heard the door bell ring.
as
主从句同时发生、
一边…一边…、随着
持续性动词
As time went on, he grew more and more impatient.
As we walked, we talked.
while
主从句同时发生
然而、既然
持续性动词
While she was reading a novel, her mother was cooking the dinner.
Some people waste food while other haven’t enough.
—I’m going to the post office. 
—While you’re there, can you get me some stamps?
4、巧记10个瞬间性动词
在学习以when和while引导的时间状语从句时，用到了瞬间动词和延续动词。常见的瞬间动词有
die(死)，come(来)，go(去)，eave(离开)，arrive(到达)，begin(开始)，buy(买)，,join(加入)，borrow(借进，借出)，return(归还)等。

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