时间状语从句
   在复合句中，由时间连接词引导的状语从句叫做时间状语从句。(在复合句中，要注意主句和从句的时态大多都要保持一致。）
   时间状语从句，由以下连词引导：
1.when在......的时候
   Mozart started writing music when he was four years old.
2.while在......期间
   He visited a lot of places while he was traveling.
3.as在......的同时；一边......一边......
   We always sing as we walk.
4.as soon as 一……就……
   We began to work as soon as we got there.
5.since 自……以来 到现在
   表示自过去的一个起始时间点到目前(说话时间)为止的一段持续时间。主句一般用现在完成时，从句用一般过去时。还可以用作介词，本句从句还可以用短语：since three years ago(从三年前至今)表示。
Mr. Green has taught in that school since he came to China three years ago.
6． till /until直到……
    都可以作连词，连接时间状语，也可以作介词，与其它词构成介词短语，在句中作状语。（强调将一般用until）
They walked till /until it was dark.

时间状语从句注意事项
when, while和as的区别
   when引导的从句的谓语动词可以是延续性的动词，又可以是瞬间动词。并且when有时表示“就在那时”。 例如： 
   When she came in, I stopped eating. (瞬间动词)
   When I lived in the countryside, I used to carry some water for him.（延续性的动词）
   While引导的从句的谓语动词必须是延续性的，并强调主句和从句的动作同时发生（或者相对应）。并且while有时还可以表示对比。例如：
   While my wife was reading the newspaper, I was watching TV.(was reading是延续性的动词，was reading和was watching同时发生)
   I like playing football while you like playing basketball.（对比）
   as表示“一边……一边”，as引导的动作是延续性的动作，一般用于主句和从句动作同时发生；as也可以强调“一先一后。例如：
We always sing as we walk.（as表示“一边……一边”）
As we was going out, it began to snow.（as强调句中两个动作紧接着先后发生，而不强调开始下雪的特定时间）
as, when, while的辨析
   as when while都表示主、从句动作同时发生，三者差异如下：

as
   表示“一边。。。一边"的意思
   as 强调两个动作同时进行，并表示对比时用于发生时间较短时

when
  1、还可以表示从句动词的动作在主几句动词的动作"之前 "或"之后"发生。 
  2、when=and then; at that moment (正在那个时候)

while
  1、用于时间较长时 
  2、强调两个动作同时进行，并表示对比时有时这三个连词可以互换，有时不可以。
   It was raining hard when (as) I got there.
(动作同时发生，when可换为as, 但不能换为while，因为get是点动词.)
   When I had read the article, he called me.
( 从句动作发生在主句之前，注意时态表达，只能用when )
   When I got to the cinema, the film had begun.
(从句的动作发生在主句之后，只能用when,并要注意时态)
   He was about to leave, when the telephone rang.
(此时不能放在句首。主句动词一般表达 “正在” “即将”, while, as不能代替)
   She thought I was talking about her daughter, while, in fact, I was talking about my daughter. (表转折，对比，when, as都不能代替它)
   While the alien was buying a souvenir, the girl called the police.
(表示主句，从句的动作同时发生，while后引导的状语从句的动词必须是延续动词不能是点动词，因为它表示较长时间)
   Mother was worried because little Alice was ill, especially as (when/ while) father was away. (此时as ,when, while可通用)

由till或until引导的时间状语从句
   till和until一般情况下两者可以互换，但是在强调句型中多用until。并且要注意的是：如果主句中的谓语动词是瞬时动词时，必须用否定形式；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。例如：
   I didn't go to bed until（till） my father came back.
   It was not until the meeting was over that he began to teach me English.
   I worked until he came back.
   I didn’t work until he came back.
   Please wait until I arrived.

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。例如：
    I have been in Beijing since you left.
    It is four years since my sister lived in Beijing.