一、as引导状语从句
1，引导时间状语从句，意思是当…的时候"，同when/while，更强调“一边 一边…."。
表“随.…的意思时同with，但with是介词，不可以跟从句。
强调主句和从句中的动作或状态同时发生
从句可放在主句前，也可放在主句后。
一边做。。。。一边做。。。。
eg：I saw him as he was getting off the bus.
He often listens to the music as he is doing his homework.
As time goes by，his illness is getting worse.
As I waited at the stop，I heard a big noise.
当我在车站等车的时候，我听到一个很大的声响。
She rose up as he entered.
当他进来时，她站了起来。
2、原因状语从句，同since。
其语气不如because强；通常为附加说明的理由，且是已知晓的原因；主句与从句没有逻辑上的因果关系。
大家都知道因为。。。
eg：We had better hurry as it is getting dark.
As she has no car，she daget there easily.
As she was not well，I went there alone.
因为她身体不好，所以我独自到那里去了。
As he is a tailor，he knows what to do with this material.
由于他是个裁缝，他知道怎样利用这块布
料。
3、方式状语从句表示“按照”、“依照”、“像”
从句置于主句之后。
You should do as the teacher tells you.
你应当依照老师所说的去做。
The absence of air also explains why thestars do not seem to twinkle in space as they do from the earth.
空气的不存在，也说明了为什么在空中星星看来并不闪烁，而不像从地球上看的那样。
4、as表示“像 一样”，引导比较状语从句
对主句和从句的动作或状态加以比较，说明它们之间有相似之处；在从句中常省略与主句相同的部分。
前一个as是副词，后一个as是比较状语从句的连词，宾语重复，可删除。
第一个 as 也可换成 so。
He doesn’t work as hard as I（do）.
他不像我工作那样努力。
.I hope the necklace was as good as the one you lent me.
我希望我挂项链同你借给我的一样好。
You’ve made as many mistakes as I have.
你犯的错误和我犯的一样多。
I haven’t got as much money as I thought.
我不像原来想像的有那么多钱
“as +形容词+a(n)+单数名词+as"。如：
She is as good a teacher as her mother. 她和她妈妈一样是位好老师。
也可说。如：She is a teacher as good as her mother.
但不说。如：She is as a good teacher as her mother. / They are as good teachers as us.
This is as good an example as the other is.
He is as hard-working a boy as anyone else in his class.他跟班上其他的任何一个同学一样,是个勤奋的学生
5、As引导让步状语从句倒装 https://www.jianshu.com/p/0451d3a162ed
名词+ as/though+主语+动词（系动词）
【说明】其中的动词通常为连系动词，也就是说位于句首的名词是其后连系动词的表语。另外，要特别注意位于句首的名词前习惯上不用词，即使其前有形容词修饰也不用冠词。比较：
King as he was，he was unhappy.
他虽是国王，但他并不幸福。
Child as he is，he knows to help other.
他虽是个孩子，却知道帮助别人。
二、as引导定语从句
1，as引导限制性定语从句。
as作为关系代词可以引导限制性定语从句，
the same…as
as…as
such…as （那些…和…一样的）
so…as
主句中出现the same，as，such，so修饰先行词，需选择as做关系代词在定语从句中做主语或宾语。
It’s the same person as we wanted to find yesterday
我们昨天要找的是同一个人。
2.Such girls as he knows are good at English
他所认识的女孩都擅长英语。
3.Do you have such books as we like？
你有我们喜欢那种书吗？
eg：I have never heard such stories as he tells.
This is the same dictionary as I lost.
5.There is so warm a house as we want to live in.
这里有如此暖和的房子，我们都想住在里面。
6.He has so difficult a problem，as none of us can solve.
他有如此难的一个问题，我们没有人能解决。
2，as引导非限制性定语从句。
as指代整个主句内容，从句可置于句首，句中或句尾
as与which一样都可以引导非限制性定语从句，但是as引导的从句可以放在主句之前、主句之后，甚至还可以分割主句；而which引导的非限制性定语从句只能放在主句之后。另外as还有“正如、就像"之意。
非限制性定语从句中的谓语为被动式时，常用as做主语
be said/known/announced
/reported/mentionedlexpected/discussed
The boy has as much progress as we had expected.
正像我们所预料的那样，这个男孩取得了大的进步
Grammar，as has been said before，is not a set of dead rules.
语法，就像以前所说，不是一套死规则。
1.As everyone knows，China is a beautiful country with a long history
每个人都知道，中国是一个有着悠久历史的美丽国家。
2.The earth，as we know，moves round the sun.
地球，我们都知道，围着太阳转。
3.Tom works hard and is willing to help others，as we all know.
汤母工作努力，并且乐于助人，这一点我们都知道。
eg：As is mentioned above，the number of the students in senior high schools is increasing.
He was admitted to college，which made his parents very happy.
John，as you know，is a writer.
