till(until) 在时间状语从句中，要注意时态一致。一般情况下主句是将来时的时候，从句要用一般现在时。
1 when 的用法
1．当…………的时候When I was young, I went to town myself.
（当）我还年轻时，我自己独自去城镇里。
When he receives the letter, he’ll tell us.
（当）他接到信后，他会告诉我们的。
2．when = and then; at that moment（正在那个时候）
此时when 前面一般要有逗号，when 不能放在句首。
主句动词一般表达“正在，即将”。when 和suddenly 经常连用。
We were watching TV, when suddenly the light went out
我们大家正在看电视，忽然灯灭了。
He was about to leave, when the telephone rang
他正要离开，忽然电话铃响了。
必背！
表时间的状语从句，还可以用复合关系词引导。
whenever = no matter when
Whenever(=No matter when)I visited him, he was not at home.
我无论什么时候拜访他，他都不在家。
2 while 的用法
1．当…………时表示主句、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，因为它表示较长的时间。
The hero looked about himself, while the enemy was studying(studied) the map.
敌人研究（仔细查看）地图时，英雄四处看看。
He visited a lot of places while he was travelling.
★while（当…………时）和介词during 意思相近。
=He visited a lot of places during his travelling.
他在旅行途中参观了许多地方。
2．while 有时含有对比的意思。
I was doing my homework while my brother was playing games.
我做功课，而我弟弟却在玩游戏。
They are classmates. He is a doctor, while she is still studying
in a high school.
他们俩个是同班同学。可是他己经是个博士了，而她仍在高中读书。
3 as 的用法
1．在…………的同时；一边…………一边…………He smiled as he left me.
他微笑着离开了我。
As we walked, we talked.
我们边走边谈。
2 ．有时用just as 表示两件事在时间上很接近。
Just as I was leaving, the phone rang again.
我正要离开时，电话铃又响了。
比较
as, when, while 的区别
as、while 和when 都表示主、从句的动作或状态同时发生，三者差异如下：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
as ①用于发生时间较短时
②表示一边…………一边…………的意思时
when 另外还可以表示从句动词的动作在主句动词的动作“之前”或“之后”发生。
所以有时三个词可以互换，有时不可以。
It was raining hard when (as) I got there.
我到达那里时，正在下大雨。
（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.
我看完这篇文章后，他给我打了电话。
（从句动作发生在主句之前，注意时态的表达，只能用when）
When I got to the cinema, the film had begun.
（当）我到了电影院时，电影已经开演了。
（从句的动作发生在主句之后，只能用when，并要注意时态。）
She thought I was talking about her daughter, while, in fact,
I was talking about my daughter.
她以为我在谈论她女儿，然而实际上我在谈论我的女儿。
（while 表转折、对比、when, as 都不能替代它）
Mother was worried because litte Alice was ill, especially
as(when/while) Father was away in France.
妈妈担心，因为小艾丽思病了，特别是当她父亲离家在法国的时候。
（此时as, when, while 可通用）
4 since 的用法
1．当“自…………以来，到现在”解表示自过去的一个起点时间到目前(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。
I have worked here since I graduated from the university thirty years ago.
我自从三十年前大学毕业以来，就在这儿工作。
（since 还可以作介词，本句since 从句还可以用短语： since
thirty years ago（自从三十年前以来）表示。）
He has been ill since he went to England last month.
自从他上个月到英国后，他就病了。
2．常用句型 It + is ……since……
It is many years since World War Two.
or: It is many years since World War Two was over.
自从二次大战（结束）以来，已经很多年了。
How long is it since you were a P．L．A．man?
=How long have you been a P．L．A．man?
你参军多久了？
注意 因为“since + 从句或名词”表示的是一段时间，所以现在完
成时的动词要用延续动词。
5 after ,before 的用法
1．after:在…………之后；before:在…………之前Do it now before you forget.
趁你还没忘，现在就去做。〈注意翻译〉
After he had left school, he joined the army.
（毕业之后）离开学校之后，他参了军。
2．常用句型It is （was）（not） +时间+before+从句
用了多长时间…………
It was not long before I fotgot it all
没过多久我就把它全忘了。
It was long （one year）before she came
她过了很久（过了一年）才回来。
6 as soon as:一…………就…………
和它表达意思一样的还有no sooner……than……；hardly……when……；scarcely……when……（请参阅
本书倒装句部 分）及单词directly, immediately, the moment 等
As soon as/Immediately/Directly he came home, I told him about that.
他一回家我就告诉他那件事。
He’ll go with you as soon as he finishes his work.
他一完成他的工作，他就和你一起去。
7 till （until）直到…………才
We walked and walked until it was dark.我们一直走，走到黑为止。