状语从句是一个句子在复合句中充当状语，修饰主句中的动词，形容次或副词等。状语从句放在主句之前时，常用逗号隔开；放在主句之后，一般不用逗号隔开。状语从句按照其意义和作用可分为时间条件原因让步目的结果方式比较地点等九种。今天我们讲时间状语从句。
时间状语从句中用一般现在时代替一般将来时。例如：
I’ll go on with the work when I come back tomorrow.
我明天回来后将继续我的工作。
时间状语从句常用连词有:when, as,while, before, after, since, till (until), as soon as, hardly ... when..., no sooner... than..., the moment, the minute, the instant等。
1） When,while, as引导时间状语从句的区别
when既可以表示时间点又表示时间段，while表示时间段和对比转折，as强调两个动作同时发生。
例如
When (不能用while) I got off the bus I fell down.当我下车时，我摔倒了。
When(=while) I was in China I visited many places of interest.(时间段）
我在中国时，我访问了许多名胜。
I was watching TV when（不能用while) he came in. (时间点）我正在电视，这时他进来了。
There is a little rain in the north while there is much rain in the south. (对比转折）
北方雨少而南方雨多。
Why can hens hatch eggs while I can’t？(对比转折）
为什么母鸡能下蛋而我不能？
As he walked along the street, he sang happily.（表示动作同时发生）
他一边走，一边高兴地唱。
As time went by, things seemed to get better. 随着时间的流逝，情况变得好多了。
As years have passed, the city has changed beyond recognition. 几年的时间过去了，这个城市已经变得让人认不出来了。
2） 表示“一…就…”的句式有：as soon as, directly (when), immediately, the moment (that), the instant (that), the minute (that), no sooner ... than，hardly...when, scarcely...when等。
No sooner had they heard the news than they rushed out into the street.
他们一听到这个消息就冲上街道。（no sooner后谓语用部分倒装）
Hardly had they heard the news when they rushed out into the street.（译文同上）
（hardly副词，几乎不，后面谓语用部分倒装）
Scarcely had they heard the news when they rushed out into the street.（译文同上）
（Scarcely副词，几乎不，简直不，后面谓语用部分倒装）
The moment/minute/instant (that) they heard the the news, they rushed out into the street.
（译文同上）（名词引导时间状语从句）
（moment瞬间，时刻, instant瞬间，顷刻, minute分，分钟）
As soon as they heard the news, they rushed out into the street.（译文同上）
On hearing the news, they rushed out into the street. （译文同上）
Directly I had done it, I knew I had made a mistake.
一做完这事，我就知道我做错了。
You may leave immediately he comes.
他一来你就可以离开了。
３）连词until 引导时间状语从句时，如果主句谓语是延续性动词，则主句用肯定式；如果谓语动词是非延续性动词，则主句用否定式。例如：
He worked until it was dark.他一直工作到天亮。
I didn’t manage to do it until you had explained how.直到你解释了以后我才会做。
Not until you had explained how did I manage to do it.（译文同上）
It was not until you had explained how that I managed to do it.（译文同上）
3） since引导的时间状语从句，如果从句中谓语动词是瞬间动词，则安字面意思翻译；如果从句中谓语动词是状态性动词（am,is, are, live, be等)或延续性动词(sing, read等）则按相反意思翻译。例如
It is (has been) 5 years since I left my hometown.(left是短暂性动词）
自我离开家乡以来，已经是5年了。
It is 5 years since I was ill.我病好以来已经有5年了。(was是状态性动词）
It is 5 years since I smoked. 我戒烟已经5年了。（smoke是延续性动词）
It is 5 years since I lived in Shanghai. 我离开上海已经5年了。
(live是状态性动词）
4） before, when, after, until等的相互转换。例如
When the bell rang, we hadn’t finished our work.
The bell rang before we had finished our work.
=铃响时，我们还没干完工作。
As soon as they finished breakfast, they ran out to play.
=They ran out to play after they finished breakfast.
他们一吃完晚饭就跑出去玩了。
When I reached the station, the train had already left.
= I reached the station, the train had already left.
= I didn’t reach the station until the train had left.
=The train had left before I reached the station.
当我到车站时，火车已离开了。
5） 尤其要注意before的汉译。例如：
Think it before you do. 三思而后行。
Three years had passed before I knew it.不知不觉间，三年已过去了。
He ran by before I could catch him. 我还没来得及抓住他，他就跑过去了。
6） long before意思是“早在…之前”，也可以引导时间状语从句。例如
I knew Betty long before you got married to her.早在你和贝蒂结婚之前，我就认识她了。
7） by the time为连词词组，引导时间状语从句,意为“不迟于，到…时为止”，所接从句常用一般过去时，主句常用过去完成时。
By the time I got to the office, the meeting had been over.当我到达办公室时，会议已经结束了。
By the time I got outside, the bus had already left.当我出来时，公共汽车已经开走了。
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