在高三课本第十五单元出现了下面几个句子：
1 As Kofi Anna，Secretary General of the United States said，“each contribution -- how small -- can help make a difference.”
2 Now she is spending a year in remote part of southwest China working as a volunteer teacher in a village school.
这几个句子都涉及到了as的用法。作介词用，第一句话中as用作关系代词，引导定语从句，含义为“正如”，第二句话中as的含义为“作为，当作”，做介词。
下面让我们结合历年高考试题对其用法进行分析。
一.用作连词引导让步状语从句
从句必须用倒装语序，把作表语用的名词(不加冠词)或作表语的形容词，或动词(原形)，或副词提到as前面。如：
1.Child as he is，he knows a lot.
2.Tired as they were，they stayed up late preparing for the coming exams.
3.Try as he would，he failed again this time.
考例：1.(2005重庆)___，he talks a lot about his favorite singers after class.
A.A quiet student as he may be
B.Quiet student as he may be
C.Be a quiet student as he may be
D.Quiet as he may be a student
【解析】答案是B。作表语的是名词student，名词前不加冠词。
2.(2005.广东)____，Carolina，couldn’t get the door open.
A.Try as she might B.As she might be C.She might as try D.Might she as try
【解析】答案是A。as引导让步状语，将动词原形try提到as前面。
二.用作关系代词，引导定语从句
as用作关系代词，引导定语从句，含义为“正如”。如：
1.He is very careful，as his work shows.
2.As is known is reported in the newspaper，talks between the two countries are making progress.
考例：(2005浙江)___I explained on the phone，your re quest will be considered at the next morning.
A.When B.After C.As D.Since
【解析】答案是C。意思是“正如我在电话里解释的那样……”。
2.(2006)The Beatles，____many of you are old enough to remember，came from Liverpool.
A.what B.that C.how D.as
【解析】答案是D。引导非限制性定语从句。
三.作为副词
用于表示“和……一样，”的意思，as…as中间要用形容词/副词的原级，如as long as：长达;as far as远达;as much as：多达;as tall as含义为“高达…”。如：
That day they walked as far as ten kilometers in two hours.
考例：(2005安徽)John is the tallest boy in the class，_______according to himself.
A.five foot eight as tall as B.as tall as five foot eight C.as five foot eight tall as D.as tall five foot eight as
【解析】选B.as…as中间要用形容词/副词的原级。
四.引导时间状语从句
含义为“当…的时候”，如：
1.I watched her as she combed her hair.
2.As he grew older ，he lost interest in everything except gardening.
考例：1.(NMET1994)Mother was worried because little Alice was ill，especially____Father was away in France.
A.as B.that C.during D.if
【解析】答案是A.“在……时候”，相当于when.而C选项是介词，不接句子。
2.(MET90)_____the day went on，the weather got worse.
A.With B.Since C.While D.As
【解析】答案是D.“随着”的意思，as the day went on可以作为固定结构记住。
五.作介词用
有“作为，当作”的意思。如：
1.They were all dressed as clowns.他们都打扮得如小丑。
考例：(2005广东)I feel that one of my main duties_____a teacher is to help the students to become better learners.
A.for B.by C.as D.with
【解析】选C。“作为教师”的意思。
