第1个回答 2010-07-12
before 在...之前,常用于过去时
since 自从,后跟表过去的时间.主语常用完成
when常用于过去时,一般跟短暂动作
while 过去,一般跟可持续性动作,用过去进行时
第2个回答 2010-07-12
before与after是一对词，分别表示在某个特定时间点的“之前”和“之后”；since表示的是“自从什么时候开始”，一般会与完成时连用；when和while，都是表示“当什么时”，区别是when既可以和延续性动词连用也可以和短暂性动词连用，而while就只能和延续性动词连用，值得注意的是while还可以表示“然而”。举个例子吧，I was watching TV when the doorbell rang.
I was playing football when/ while you were playing ping-pong.（这里用when和while是都可以，但是表示的意思路有差别哈。while多了点比较的意味。）至于as多表示“随着...的发展”。这谢是偶的个人的心得，希望对你有所帮助！(*^__^*)
第3个回答 2010-07-12
as
副词
1.跟...一样地,同样地
Sam is as rich as Alexander. 山姆像亚历山大一样富有。
2.如同;例如
Such animals as foxes and squirrels have bushy tails. 有些动物,如狐和松鼠,有蓬松的尾巴。
介词
1.作为,以...的身份
She works as an interpreter in that company. 她在该公司里担任翻译员。
2.当作
She didn't think much of him as a painter. 她对他作为一位画家评价不高。
3.像;如同
The old woman was dressed as a young lady. 这位老妇人打扮得像一位年轻人。
连词
1.像...一样;依照;像
You ought to do as Paul tells you. 你应按照保罗吩咐的做。
The work is not so difficult as you imagine. 这工作不像你想像的那么困难。
2.当...时
As she left the room she remembered that book. 她离开房间时想起了那本书。
3.随着
As the sun rose the fog dispersed. 太阳一出来,雾随之消失。
4.因为
We didn't know what to do as we were just visiting there. 我们不知道该怎么办,因为当时我们仅仅在那里作访问。
5.虽然
Tired as he was, he sat up late. 他虽然疲倦,可仍然很晚才睡。
when
副词
1.(用作疑问副词)什么时候,何时
I'd like to know when they'll let him out. 我想知道他们什么时候放他。
When did you last see Margaret? 你上一次见到玛格丽特是什么时候?
(2.用作关系副词,引导关系从句)当...时
There were times when I didn't know what to do. 有些时候我不知道该怎么办才好。
连词
1.当...时
John got engaged to her when travelling last winter. 去年冬天旅行时,约翰与她订了婚。
Don't get excited when you talk. 你说话时别激动。
2.既然,考虑到
How can he get the promotion when his boss dislikes him? 既然上司不喜欢他,那他怎么会获得提升呢?
3.虽然,然而
I had only fifteen dollars on me when the book I wanted cost twenty. 我身上只带了十五元钱,可是我想买的那本书定价二十元。
4.在那时,然后
代词
1.什么时候,那时
Since when has she taught here? 她从什么时候开始在这儿教书的?
名词
1.(事件发生的)时间
before
介词
1.在...以前
Only two days remained before the examination. 考前只剩下两天了。
2.(指顺序、重要性等)在...之前,先于
Your name comes before mine. 你的名字在我之前。
Pride comes before a fall. 骄必败。
3.由...考虑中
We will put the matter before the committee. 我将把此事提交委员会考虑。
4.在...前面;在...面前
She said she had never spoken before an audience. 她说她从未在大庭广众讲演过。
副词
1.较早
Come at two o'clock, not before. 两点钟来,不必再早了。
2.以前;曾经
He was never late before. 过去他从不迟到。
Have you two met before? 你们两人以前见过面吗?
3.在前;向前
Several soldiers went before to scout. 数名士兵在前面探路。
连词
1.在...以前
Check it carefully before you hand it in. 交来之前仔细核对一下。
2.与其...宁可
I'd die before I'd tell. 我宁死也决不会说出来。
3.否则,要不然
Get out before I call the police. 滚开,否则我叫警察。
since
副词
1.此后;从那时到现在
She left school three years ago and has worked as a nurse ever since. 她三年前毕业,从那时起便一直当护士。
2.之前,以前
介词
1.(常用于现在完成时或过去完成时)自...以来,从...至今
I haven't heard from him since last year. 我自去年以来未曾收到过他的信。
连词
1.(常用于现在完成时或过去完成时)自...以来,从...至今
It's been ten years since they married. 他们结婚到现在已经十年了。
She has been living a hard life since her husband died. 她自从丈夫死后,一直过着艰苦的生活。
2.既然;因为,由于
He must have taken the book since it isn't here. 他一定是把书拿走了,因为书已不在这里了。
Since it is so hot, let's go swimming. 既然天气这么热,我们去游泳吧。