用表示时间的连词连接一个句子作状语，这样的主从复合句就是时间状语从句。
一、when，While，as引导的时间状语从句
例句：
When you think you know nothing, then you begin to know something.
当你以为自己一无所知的时候，你就是在开始知道一些事物了。
Strike while the iron is hot. 趁热打铁。
You can feel the air moving as your hand pushes through it.
当你的手在空气中挥动的时候，你就能感觉到空气在流动。
Our headmaster laughed as she spoke.
我们的校长边谈边笑。
二、表“一……就”的时间状语从句
例句：
I will go there directly I have finished my breakfast.
吃完早饭，我立即到那里去。
The moment I heard the news, I hastened to the spot.
我一听到消息，马上赶到了出事地点。
As soon as I reach Canada, I will ring you up.
我一到加拿大，就给你来电话。
注意：hardly(scarcely, rarely)…when / before, no sooner…than相当于as soon as之意。主句用过去完成时，从句用一般过去时。当hardly, scarcely, rarely和no sooner位于句首时，主句应用倒装语序。例句：
He had no sooner arrived home than he was asked to start on another journey.
他刚到家，就被邀请开始另一旅程。
No sooner had the sun shown itself above the horizon than he got out of bed to commence work.
太阳刚从地平线上升起，他就起床劳动去了。
Hardly had I sat down when he stepped in.
我刚坐下，他就进来了。
He had hardly fallen asleep when he felt a soft touch on his shoulder.
这个阿拉伯人刚要入睡就感到肩膀上被轻轻一触。
三、till，until，not…until
例句：
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。till不可以用在句首，而until可以放在句首。
例如：
I didn't go to bed until（till） my father came back.
直到我父亲回来我才上床睡觉。
It was not until the meeting was over that he began to teach me English.
直到散会之后他才开始教我英语。
I worked until he came back.
我工作到他回来为止。
I didn't work until he came back.
他回来我这才开始工作。
Please wait until I arrived.
在我到达之前请等我。
四、before和since
1. 注意before引导的从句不再用否定式的谓语，并且当before引导的从句位于主句之后，有时译成“就，才”。还要注意主句和从句之间的时间关系。当主句用将来时，从句总是用现在时；如果before引导的主句谓语用的是过去完成时，则从句动词多用一般过去时，这样以便体现动作发生的先后。
例句：
It will be four days before they come back.
他们要过四天才能回来。
Einstein almost knocked me down before he saw me.
爱因斯坦几乎把我撞倒才看到我。
My father had left for Canada just before the letter arrived.
我父亲恰好在信到之前去加拿大了。
2. 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。
例句：
I have been in Beijing since you left.
自从你离开以来，我一直在北京了。
Where have you been since I last saw you?
自上次我和你见面以后，你到哪里去了？
It is four years since my sister lived in Beijing.
我妹妹不在北京住有四年了。
难点引导的时间状语从句的主从复合句中，动作发生在先的用过去完成时，在后的用过去时。表示前面的叙述所没有提到过的信息。它总是对所描述的事件予以引人注目的强调，意为“这时；突然”；当位于句末的分句具有引种用法时，大致有以下三种情况：
1. when 分句前面的分句使用过去进行时。如：
1) He was still smiling when the door opened and his wife came in.
他正笑着的时候，门突然
开了，他妻子走了进来。
2) I was thinking of this when I heard my name called.
我正想着这件事时，突然听到有人叫我的名字。
2. when 分句前面的分句使用 was （were） about to， was （were） on the point 等。如：
1) We were about to start when it began to rain.
我们刚要出发就开始下雨了。
2) He was on the point of leaving when someone knocked at the door.
他刚要走就有人敲门。
3. when 分句之前的分句用过去完成时，其中 sb had hardly / scarcely /barely … when已成为固
定词组。如：
1) We had hardly fallen asleep when the bell rang.
我们刚刚入睡，铃声就响了起来。
2) He had scarcely arrived when he had to leave again.
他刚刚到达就又要离开了。
易错点when, while和as的区别
1. when引导的从句的谓语动词可以是延续性的动词，可以是瞬时动词。并且when有时表示“就在那时”。
例如：When she came in, I stopped eating.
她进来时，我停止吃饭。(瞬时动词)
When I lived in the countryside, I used to carry some water for him.
当我住在农村时，我常常为他担水。（延续性的动词）
We were about to leave when he came in.
我们就要离开，就在那时他进来了。
2. While引导的从句的谓语动作必须是延续性的，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.
我喜欢踢足球，而你喜欢打篮球。（对比）
3. 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.我们总是边走边唱。（as表示“一边……一边”）
As we were going out, it began to snow.
当我们出门时，开始下雪了。（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）
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