一、as引導狀語從句
1，引導時間狀語從句，意思是當…的時候"，同when/while，更強調“一邊 一邊…."。
表“隨.…的意思時同with，但with是介詞，不可以跟從句。
強調主句和從句中的動作或狀態同時發生
從句可放在主句前，也可放在主句後。
一邊做。。。。一邊做。。。。
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.
當我在車站等車的時候，我聽到一個很大的聲響。
She rose up as he entered.
當他進來時，她站了起來。
2、原因狀語從句，同since。
其語氣不如because強；通常爲附加說明的理由，且是已知曉的原因；主句與從句沒有邏輯上的因果關係。
大家都知道因爲。。。
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.
因爲她身體不好，所以我獨自到那裏去了。
As he is a tailor，he knows what to do with this material.
由於他是個裁縫，他知道怎樣利用這塊布
料。
3、方式狀語從句表示“按照”、“依照”、“像”
從句置於主句之後。
You should do as the teacher tells you.
你應當依照老師所說的去做。
The absence of air also explains why thestars do not seem to twinkle in space as they do from the earth.
空氣的不存在，也說明了爲什麼在空中星星看來並不閃爍，而不像從地球上看的那樣。
4、as表示“像 一樣”，引導比較狀語從句
對主句和從句的動作或狀態加以比較，說明它們之間有相似之處；在從句中常省略與主句相同的部分。
前一個as是副詞，後一個as是比較狀語從句的連詞，賓語重複，可刪除。
第一個 as 也可換成 so。
He doesn’t work as hard as I（do）.
他不像我工作那樣努力。
.I hope the necklace was as good as the one you lent me.
我希望我掛項鍊同你借給我的一樣好。
You’ve made as many mistakes as I have.
你犯的錯誤和我犯的一樣多。
I haven’t got as much money as I thought.
我不像原來想像的有那麼多錢
“as +形容詞+a(n)+單數名詞+as"。如：
She is as good a teacher as her mother. 她和她媽媽一樣是位好老師。
也可說。如：She is a teacher as good as her mother.
但不說。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個同學一樣,是個勤奮的學生
5、As引導讓步狀語從句倒裝 https://www.jianshu.com/p/0451d3a162ed
名詞+ as/though+主語+動詞（系動詞）
【說明】其中的動詞通常爲連繫動詞，也就是說位於句首的名詞是其後連繫動詞的表語。另外，要特別注意位於句首的名詞前習慣上不用詞，即使其前有形容詞修飾也不用冠詞。比較：
King as he was，he was unhappy.
他雖是國王，但他並不幸福。
Child as he is，he knows to help other.
他雖是個孩子，卻知道幫助別人。
二、as引導定語從句
1，as引導限制性定語從句。
as作爲關係代詞可以引導限制性定語從句，
the same…as
as…as
such…as （那些…和…一樣的）
so…as
主句中出現the same，as，such，so修飾先行詞，需選擇as做關係代詞在定語從句中做主語或賓語。
It’s the same person as we wanted to find yesterday
我們昨天要找的是同一個人。
2.Such girls as he knows are good at English
他所認識的女孩都擅長英語。
3.Do you have such books as we like？
你有我們喜歡那種書嗎？
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.
這裏有如此暖和的房子，我們都想住在裏面。
6.He has so difficult a problem，as none of us can solve.
他有如此難的一個問題，我們沒有人能解決。
2，as引導非限制性定語從句。
as指代整個主句內容，從句可置於句首，句中或句尾
as與which一樣都可以引導非限制性定語從句，但是as引導的從句可以放在主句之前、主句之後，甚至還可以分割主句；而which引導的非限制性定語從句只能放在主句之後。另外as還有“正如、就像"之意。
非限制性定語從句中的謂語爲被動式時，常用as做主語
be said/known/announced
/reported/mentionedlexpected/discussed
The boy has as much progress as we had expected.
正像我們所預料的那樣，這個男孩取得了大的進步
Grammar，as has been said before，is not a set of dead rules.
語法，就像以前所說，不是一套死規則。
1.As everyone knows，China is a beautiful country with a long history
每個人都知道，中國是一個有着悠久歷史的美麗國家。
2.The earth，as we know，moves round the sun.
地球，我們都知道，圍着太陽轉。
3.Tom works hard and is willing to help others，as we all know.
湯母工作努力，並且樂於助人，這一點我們都知道。
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.