北外招生办:如何区分as,while,since,until

as表示”一边．．．．．．一边．．．．．．”
We sang songs as we went out for a walk.

while引导的从句中动词必须是延续性的；在“be…when…”句式中when表“at that time(就在这时)”意，这样用的when不能换为while；while有时并不表示时间，而表示对比，意“而”、“却”，when无这样的用法。例如：When I got home I found the door locked./While(或When)we were working in the fields,it suddenly began to rain./He was wandering through the streets when a bike hit him./His pencil is red,while mine is yellow.

since的四种用法

1) since +过去一个时间点 (如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.

2) since +一段时间+ ago
I have been here since five months ago.

3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.

4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.

Until直到….为止, not until直到…才

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。
（在肯定句中可用before代替 Let”s get in the wheat before the sun sets.）

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。

否定句可用另外两种句式表示。
(1)Not until …在句首，主句用倒装。
Not until the early years of the 19th century did man know what heat is.
直到19 世纪初，人类才知道热能是什么。
Not until I began to work did I realize how much time I had wasted.
直到我开始工作，我才认识到了我已蹉跎了几多岁月。
(2) It is not until… that…这是一个强调结构。如本题。

Till的用法基本相同，但

1)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。
2)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。

since prep.自从，自……以来（表示时间，后接名词或从句）
until prep.直到，到……为止
直到……（才），在……前（用于否定句）
as prep. 作为，当作
像…… 如…… 似……
while prep. 直到
conj. 表示“同时”两个动作同时进行