as可以用作连词，副词，介词，关系代词，是极其重要的高考考点。
1.as conj
①当...时候，一边...一边，随着
He sang songs as he was cutting grass.他一边割草一边唱歌。
As the society develops, our life is getting better and better.随着社会的发展，我们的生活越来越好。
②因为，既然
As he was ill, we decided to go without him.因为他病了，我们决定不带他去。
❊重点提示：because, since, as, for表原因的区别
❆because语气最强，常用于why引导的问句。
--Why didn't you go to the party last night?你昨晚为什么没去参加聚会?
--Because I have something urgent to do.因为我有急事要做。
❆since比because语气弱，常表示对方已知的原因，意思是“既然”
Since everyone is here, let's begin our class.既然大家都到齐了，我们开始上课吧。
❆as与since相似，是明显的或已知的原因，可互换，意思是“由于”。
As you were out, I left a message.因为你不在，我留了一张字条儿。
❆for为并列连词，引导并列分句，语气最弱，表示补充说明的原因。
He must have left, for the light were out.他一定是走了，因为灯都灭了。
③尽管，虽然（引导让步状语从句）
Young as he is, he can speak several languages.他虽然年轻，却能说好几种语言。
❊重点提示：as，though，although，while引导让步状语从句的区别
❆as引导的让步状语从句要用倒装语序，结构为：形容词/名词/副词/动词原形 as 主语 谓语，当名词提前时不加冠词。
Clever as she was, she worked very hard.尽管她很聪明，但她工作很努力。
Child as he is, he set a good example for us.他虽然是个孩子，却给我们树立了好榜样。
Try as he might, he failed the exam again.尽管他尽力了，他考试还是不及格。
Much as I love you, I can't marry you.虽然我很爱你，但我不能嫁给你。
❆though引导的让步状语从句可用倒装语序，也可不用。
❆although，while引导的让步状语从句不能用倒装语序，while引导的从句常放在句首。
While/Although online shopping has changed our life, not all of its effects have been positive.虽然网上购物改变了我们的生活，但并非所有的影响都是积极的。（2016浙江卷）
④像，如同，按照
When in Rome, do as the Romans do.入乡随俗。
Leave my books as they are when I leave.我走的时候，让我的书保持原样。
2.as adv 用于形容词或副词前面，as...as像...一样，如同
He doesn't earn as much as me.他挣得没有我多。
Please run as fast as you can.请尽可能快地跑。
3.as prep
①作为
He worked as a teacher in the high school.他在那所高中当老师。
As a student, we should work hard.作为一名学生，我们应该努力学习。
②像，如同
The boys were all dressed as clowns.男孩们都打扮成小丑。
4.as pron 正如
①such/the same...as，as为关系代词，引导限制性定语从句，在从句中作主语，宾语，表语。
He is not such a poor man as he looks.他并不像看上去那么穷。
The blouse you bought is not the same as I have.你买的衬衫和我的不一样。
②作关系代词，引导非限制性定语从句，as指代整个句子的内容。
As we all know, music plays an important part in our life.众所周知，音乐在我们的生活中扮演着重要的角色。
❊重点提示：as 和which作关系代词，指代整个句子的内容，引导非限制性定语从句的区别：
as意思为“正如”，as引导的定语从句位置灵活，可位于句首，句中，句尾。
which意思为“这，这一点”，which引导的定语从句只能放在句末。
He failed in the exam, which made his parents very worried.他考试不及格，这使他的父母非常担心。
