①  when引导的时间状语从句

a.       when引导的时间状语从句，意为“当…时”，表示主句的动作和从句的动作同时或先后发生。

When you are crossing the street, you must be careful.

I was sleeping when someone knocked at the door.

b.       when引导的时间状语从句中的动词可以用延续性的动词，也可以用终止性的动词。

He was working at the table when I came in.

I worked for a foreign company when I was in Shanghai.

②  while引导的时间状语从句

    while引导的时间状语从句，常意为“与…同时； 在…期间”，while引导的从句常用延续性动词或状态动词。

    They rushed in while we were discussing the problem.

    We must strike while the iron is hot.

③      as引导的时间状语从句

as引导的时间状语从句，意为“正当； 一边…一边； 随着”等意思。

I heard the murmur of their voices as I crossed the hall.

As she sang, tears ran down her cheeks.

④      before引导的时间状语从句，意为“在…之前”，表示主句的动作发生在从句的动作之前。

We cleaned the classroom before we left school yesterday.

Ø         It is some time before … 这个句型表示“在…之前就有一段时间了”。

    It will still be a long time before he comes back.

⑤      after引导的时间状语从句

after引导的时间状语从句，意为“在…之后”，表示主句的动作发生在从句的动作之后。

He called me after he had finished his work.

⑥      since引导的时间状语从句

since引导的时间状语从句，意为“自从…”，主句常用现在完成时， 从句常用一般过去时。

He has worked in this city since he graduated from the college.

Since it begun in 1951, we have given awards to films from all around the world.

a.  It is/has been some time since … 这个句型表示“自从以来…有多长时间了”。since与短暂性动词连用时，表示动作的开始，since与延续性动词连用表示该动作或状态的结束，翻译时通常译成该动词的反义词。

e.g.   It is/has been six years since she graduated from the university.

      It is five years since I lived in Nanchang. (自从我不住在南昌以来已经5年了。)

It is five years since I began to live in Nanchang. (自从我住在南昌以来已经5年了。)

It is three weeks since he was ill. 他的病已经好了三周了。

It is three weeks since he fell ill. 他已经病了三周了。

b.  since作介词时，后接时间点，不接时间段，since还可作副词用。

He has been writing the book since five years. (×)

He has been writing the book since five years ago. (√)

He has been writing the book since he retired. (√)

He has been writing the book for five years. (√)

He has been writing the book (ever) since (then). (√)

⑦      until/till引导的时间状语从句, 意为“直到…时”，表示主句的动作发生在从句的动作之前。

I’ll stay here until you come back.

He didn’t go to bed until he finished his homework.

Ø         此两个连词意义相同。肯定形式表示的意思是"做某事直至某时"，动词必须是延续性的。否定形式表

达的意思是"直至某时才做某事"。动词为延续性或非延续性都可以。

肯定句： I slept until midnight. 我一直睡到半夜时醒了。
　       Wait till I call you. 等着我叫你。
否定句： She didn't arrive until 6 o'clock. 她直到6点才到。
　       Don't get off the bus until it has stopped. 公共汽车停稳后再下车。
　       I didn't manage to do it until you had explained how. 直到你教我后，我才会做。

a.    Until可用于句首，而till通常不用于句首。
　  Until you told me, I had heard nothing of what happened.

直到你告诉我以前，出了什么事我一点也不知道。

b.    Until when 疑问句中，until要放在句首。
--- Until when are you staying?　你呆到什么时候？

--- Until next Monday.　呆到下周一。　

Ø         否定句可用另外两种句式表示。

a.