时间状语从句

一 when, while和as引导的时间状语从句

* 状语从句里的一般将来时用一般现在时表示。

1 when, while 和 as 在引导时间状语从句时都做“当......时候”解，三者往往可以互换，但要求是：主句动词是结束性动词，从句动词是延续性动词。

The boy fell asleep when(while, as) he was reading.

2 when用于主句和从句动作同时发生或从句动作先于主句动作，强调“特定时间”或“带有规律性”若主句和从句动作不是同时发生而是以“先后顺序”发生的，则只用when。

When spring comes, he feels like a trip.(特定时间）

It's cold when it snows. (规律性）

I will go home when he comes back. (先后顺序）

3 as 多用在口语中，强调“同一时间”或“一前一后”，有时还有“随着”的意思，当主句和从句的主语相同时，as有“一边......一边”的意思。

As he was eating his breakfast, he heard the door bell ring.(强调同一时间）

= When he was eating his breakfast, he heard the door bell ring.(强调特定时间）

= While he was eating his breakfast, he heard the door bell ring.(强调吃饭的过程）

Things are getting better and better as time goes on.(随着）

She sang as went along.(一边......一边）

4 while引导的时间状语从句中的动词必须是延续性的，表示的时间是“一段”，主句和从句的动作同时发生。

I was reading while he was writing a letter.

While he was doing his homework, the telephone rang.

= When the telephone rang, he was doing his homework.

5 as和when之后可直接一个名词，构成省略句；而while一般不这样用。

As(When) a boy, he lived in Japan.

6 when和while之后可以接分词、介词短语、形容词等，而as一般不这样用。

When(While)reading,he fell asleep.(接分词）

When(while) at school, he wrote his first novel.(接介词短语）

You must study hard while(you are) young, or you will regret when(you are) old.(接形容词）

7 when既可以引导状语从句，又可以引导宾语从句、定语从句等，注意区别。

Can you tell me when the next train leaves?(when引导宾语从句）

He came at a time when we needed him most. (when引导定语从句）

It was raining hard when the students got to school yesterday.（when引导时间状语从句）

二 till 和untill引导的时间状语从句

1 till和untill引导的时间状语从句，主句的谓语动词是延续性动词时，主句用肯定形式，表示这一动作或状态一直延续到till/untill所表示的时间为止。till和untill都表示“直到”，常可互换，但till不用于句首，也不用于强调句。untill比till更正式。

You may stay here till/untill the rain stops.

Untill he returns, nothing can be done.(此句不能用 till)

2 常用句型 not ......untill /till ......