时间状语从句时态

1.由when, while和as引导的时间状语从句

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，我在吃饭。(瞬时动词)
When I lived in the countryside, I used to carry some water for him.当的住在农村时，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。
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）
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 was 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）

2.由before和after引导的时间状语从句。

before:当主句用将来时，从句总是用现在时；如果before引导的从句谓语用的是过去时，则主句动词多用过去完成时，这样以便体现动作发生的先后。
After表示主句动作发生在从句动作之后。主句和从句的动作的时间关系正好与before引导的从句相反。例如：After we had finished the work, we went home.完成工作之后，我们回家了。（从句用过去完成时，主句用一般过去时）

3.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。till不可以用在句首，而until可以放在句首。例如：
I won't stop crying until you let me go.你不教我走我就一直喊叫。
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。

4.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. 自从你离开以来，我一直在北京了。
It is four years since my sister lived in Beijing. 我妹妹不在北京住有四年了。

5.由as soon as, immediately, directly, instantly, the moment, the instant, the minute, 等引导的时间状语从句。这些连词都表示“一……就”。例如：
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。例如：
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。

6.由by the time引导的时间状语从句。注意时态的变化：在一般情况下，如果从句的谓语动词用一般过去时，主句的谓语动词用过去完成时；如果从句的谓语动词用一般现在时，主句的谓语动词用将来完成时。例如：
By the time you came back, I had finished this book.到你回来时，我已经写完这本书了。
By the time you come here tomorrow, I will have finished this work. 你明天来这儿的时候，我将已经完成此工作了。

7.由each time, every time和whenever引导的时间状语从句。例如：
Each time he came to Harbin, he would call on me. 他每次来哈尔滨，总是来看我。
Whenever that man says“To tell the truth”, I suspect that he's about to tell a lie.每当那个人说“说实在话”的时候，我猜想他就要说谎了。
You grow younger every time I see you. 每次遇到你，见你更年轻了。

8.由as long as和so long as引导的时间状语从句。这两个连词表示“只要“例如：
You can go where you like as long as you get back before dark. 你可以随意到哪里去，只要在天黑以前回来就行。
I will fight against these conditions as long as there is a breath in my body! 只要我一息尚存，我就要反对这种境况。

具体你可以参考这一段。
when，while，as引导时间状语从句的区别
when，while，as显然都可以引导时间状语从句，但用法区别非常大。
一、when可以和延续性动词连用，也可以和短暂性动词连用；而while和as只能和延续性动词连用。
① Why do you want a new job when youve got such a good one already？（get为短暂性动词）你已经找到如此好的工作，为何还想再找新的？
②Sorry，I was out when you called me．（call为短暂性动词）对不起，你打电话时我刚好外出了。
③Strike while the iron is hot．（is为延续性动词，表示一种持续的状态）趁热打铁。
④ The students took notes as they listened．（listen为延续性动词）学生们边听课边做笔记。
二、when从句的谓语动词可以在主句谓语动作之前、之后或同时发生；while和as从句的谓语动作必须是和主句谓语动作同时发生。
1．从句动作在主句动作前发生，只用 when。
①When he had finished his homework，he took a short rest．（finished先发生）当他完成作业后，他休息了一会儿。
②When I got to the airport，the guests had left．（got to后发生）当我赶到飞机场时，客人们已经离开了。
2．从句动作和主句动作同时发生，且从句动作为延续性动词时，when，while，as都可使用。
①When ／While ／As we were dancing，a stranger came in．（dance为延续性动词）当我们跳舞时，一位陌生人走了进来。
②When ／While ／As she was making a phonecall，I was writing a letter．（make为延续性动词）当她在打电话时，我正在写信。
3．当主句、从句动作同时进行，从句动作的时间概念淡化，而主要表示主句动作发生的背景或条件时，只能用 as。这时，as常表示“随着……”；“一边……，一边……”之意。
① As the time went on，the weather got worse．（as表示“随着……”之意）
② The atmosphere gets thinner and thinner as the height increases．随着高度的增加，大气越来越稀薄。
③As years go by，China is getting stronger and richer．随着时间一年一年过去，中国变得越来越富强了。
④The little girls sang as they went．小姑娘们一边走，一边唱。
⑤The sad mother sat on the roadside，shouting as she was crying．伤心的妈妈坐在路边，边哭边叫。
4．在将来时从句中，常用when，且从句须用一般时代替将来时。
①You shall borrow the book when I have finished reading it．在我读完这本书后，你可以借阅。
②When the manager comes here for a visit next week，Ill talk with him about this．下周，经理来这参观时，我会和他谈谈此事。
三、when用于表示“一……就……”的句型中（指过去的事情）。
sb．had hardly（＝scarcely） done sth．when．．．＝Hardly ／ Scarcely had sb．done sth．when．．．
①I had hardly ／scarcely closed my eyes when someone knocked at the door．＝Hardly ／ Scarcely had I closed my eyes when someone knocked at the door．我刚一闭上眼，就有人在敲门了。
②I had hardly ／scarcely entered my room when the telephone rang．＝Hardly ／Scarcely had I entered my room when the telephone rang．我刚一走进房门，电话就响了。

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