as
1.引导限制性定语从句和非限制性定语从句
（1）as多与such 或the same连用,可以代替先行词是人或物的名词.
（2）as 也可单独使用,引导非限制性定语从句,作用相当于which.例如：
The elephant’s nose is like a snake,as anybody can see.
（3）the same… that与 the same …as在意思上是不同的.
2．As引导的非限制性定语从句的位置
as引导的非限制性定语从句位置较灵活,可以位于主句前面．中间或后面,一般用逗号与主句隔开,但which所引导的非限制性定语从句只能放在主句之后.例如：
(1) As is expected,the England team won the football match.
(2) The earth runs around the sun,as is known by everyone.
when
1.关系副词 when引导定语从句（包括限定性定语从句和非限定性定语从句）修饰表示时间的先行词,在定语从句中充当时间状语.例如：
I still remember the day when I first met Jennifer.
我仍然记得第一次见到詹妮弗的那一天.
Next month,when you’ll spend your summer holidays in your hometown,is approaching.
下个月即将来临,届时你们将在家乡度暑假.
as,when,while的区别
as,when,while都表示主、从句的动作或状态同时发生,但三者意义不尽相同.as和when引导的从句既可表示一点时间,也可表示一段时间,从句中的谓语动词既可以是持续性动词,也可以为短暂性动词,经常可以互换使用；while引导的从句通常表示一段时间,从句中宜用持续性动词作谓语.当从句中的谓语动词为持续性动词时,这三者可以通用（前面例句中已有体现）,再如：
Mother was worried because little Alice was ill,especially as / when / while father was away in France.妈妈担心,因为小艾丽思病了,特别是当父亲远在法国的时候.
如果从句和主句要表示一个人的两个动作交替进行或同时完成时,则多用as,可译为“一边.,一边.”.例如：
He looked behind from to time as he went.他一边走,一边不时地往后看.
As time goes on,it’s getting warmer and warmer..随着时间的推移,天气变得越来越暖了.
I thought of it just as you opened your mouth.你一张嘴我就知道你要说什么.（若表示两个短促动作几乎同时发生时,用as的场合多于when.）
当主从句之间表示转折或对比关系时,多用while,不用as或when.如：
She thought I was talking about her daughter,while in fact,I was talking about my daughter.她以为我在谈论她女儿,而事实上,我在谈论我的女儿.
如果表示从句的动作在主句的动作“之前”或“之后”发生时,多用when（=after）,不用as或 while.此外,when还含有“at that moment”的意思（= and then）,引起的句子不能放在句首,也不能用as,while来替换.例如：
He was about to leave when the telephone rang.他正要离开,忽然电话铃响了.
We were watching TV when suddenly the lights went out.我们正在看电视,突然灯灭了.
They had just arrived home when it began to rain.他们刚到家,天就开始下雨了.