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）
as when while的辨析
as when while都表示主、从句动作同时发生，三者差异如下：
as表示“一边。。。一边＂的意思
强调两个动作同时进行，并表示对比时
用于发生时间较段时
when
1、还可以表示从句动词的动作在主几句动词的动作＂之前 ＂或＂之后＂发生。
2、when＝and then; at that moment (正在那个时候)
3.常用于常见搭配中
while
1、用于时间较长时
2、 强调两个动作同时进行，并表示对比时
有时这三个连词可以互换，有时不可以。
lt was raining hard when (as) I got there.
我到那里时，正在下大雨。 ( 动作同时发生，when可换为as, 但不能换为while，因为get是点动词.)
When I had read the article, he called me.
我看完这篇文章之后，她给我打了电话。( 从句动作发生在主句之前，注意时态表达，只能用when )
When I got to the cinema, the film had begun.
(当)我到了电影院时，电影已经开演了。(从句的动作发生在主句之后，只能用when,并要注
意时态)
He was about to leave, when the telephone rang.
他正要离开，忽然电话响了。( 此时不能放在句首。主句动词一般表达 “正在” “即将”.
while, as不能代替
She thought I was talking about her daughter, while, in fact, I was talking about my daughter.
他以为我在谈她女儿，然而，实际上在谈论我女儿。(表转折，对比，when, as都不能代替它)
While the alien was buying a souvenir, the girl called the police.
外星人买纪念品时，那女孩给警察打了电话。(表示主句，从句的动作同时发生，while后引导的状语从句的动词必须是延续动词不能是点动词，因为它表示较长时间)
Mother was worried because little Alice was ill, especially as (when/ while) father was away.
妈妈担心，因为小爱丽丝病了，特别是他父亲不在家的时候。(此时as ,when, while可通用)
2. 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。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。
3. 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，表述为：现在完成时+since+一般过去时。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。
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。
It is five months since our boss was in Beijing.我们老板离开北京有五个月了。
知识扩展
1. It is since从。。。以来多长时间了(因为since +从句或名词，表示一段时间)
It is five years since we met last time.从我们上次见面已经五年了。
2. It is +before?(。。。才)
It was a long time before I went to sleep again.
过了很长时间我才睡着。
It was an hour before(＝until) the police arrived.
过了一个小时，警察才来。
二.原因状语从句：在一个句子中作时间状语的句子。
连接词：由连词because, since, as引导, 也可由for, now that 等词引导
1.I didn’t go to school yesterday because I was ill.
我昨天没去上学，因为我生病了。
2. Since everybody is here, let’s begin our meeting.
既然大家都来了, 让我们开始开会吧.
3. As you are in poor health, you should not stay up late.
既然你身体不好, 你就不该熬夜.
4.I asked her to stay to tea, for I had something to tell her.
我请她留下来喝茶,因为我有事要告诉她.
用法辨析：because , since , as , for辨析
1）because语势最强，用来说明人所不知的原因，回答why提出的问题。当原因是显而易见的或已为人们所知，就用as或 since。
I didn’t go, because I was afraid.
Since /As the weather is so bad, we have to delay our journey.
2）由because引导的从句如果放在句末，且前面有逗 号，则可以用for来代替。但如果不是说明直接原因，而是多种情况加以推断，就只能用for。
He is absent today, becaus e / for he is ill.
He must be ill, for he is absent today.
三、条件状语从句
连接词：if如果, unless (=if not) 除非。(让步)
1.If it doesn’t rain tomorrow, we will go hiking.
如果明天不下雨, 我们就去远足.
2.You will get good grades if you study hard.
如果你努力学习,就会取得好成绩.
3.I will go to the party unless he goes there too.
我不会去参加聚会的, 除非他也去.(如果他不去,我也不去.)
4.You will be late unless you leave immediately.
如果你不马上走,你将会迟到的.(=If you don’t leave immediately, you will be late.)
难点提示：用条件状语从句时要注意时态的正确使用,当主句是将来时的时候,从句要用一般现在时.
He will not leave if it isn’t fine tomorrow.
一般将来时， 一般现在时
They are going to have a picnic if it doesn’t rain next week.
一般将来时, 一般现在时
四、目的、结果状语从句
目的状语从句是指在一个句子中充当目的状语的句子。
结果状语从句是指在一个句子中充当结果状语的句子
目的状语从句连接词so that, so?that , in order that 引导。
结果状语从句连接词 so?that, such?that, so much/many?that引导。
1.so?that 如此?以至于
The scientist’s report was so instructive that we were all very excited.
科学家的报告很有教育性，我们感到很兴奋。
He always studied so hard that he made great progress.
他总是那么努力，所以他取得了很大的进步。
2.so that 以至于, 以便于
I’ll run slowly so that you can catch up with me. (目的)
我将慢慢跑以至于你能赶上我。
I opened the window so that fresh air might come in. (目的)
我把窗户打开以便于使新鲜空气可以进来。
3. such?that 如此?以至
It’s such nice weather that all of us want to go to the park.
天气是如此的好，我们大家都想去公园玩。
4.in order that=so that:为了
We shall let you know the details soon in order that you can/may make your arrangements.