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as的基本使用情况：
1、as作介词，表示作为，一般情况下后面只能接一个名词;
eg.She works as a doctor.
As a League member,I'll take the lead in everything.
作为一名团员，事事我都要身先士卒。
2、as作关系代词，as引导这样的从句一般*是放在句子的开头;
eg.The moon travels round the earth once every month which is known to everybody.
As is known to everybody,the moon travels round the earth once every month.
It is known to everybody that the moon travels round the earth once every month.(主语从句)As we have seen，the focus of medical care in our society has been shifting from curing disease to preventing disease-especially in terms of changing our many unhealthy behaviors,such as poor eating habits，smoking and failure to exercise.
3、as充当连词来构成状语从句;
①时间状语从句(当.....的时候)，eg.As the summer comes,the day becomes shorter and shorter.
②原因状语从句(表示显而易见的原因)，和since在用法上保持一致。
eg.As she was not feeling well,we all sold her to stay at home.
因为她感觉到不舒服，我们都叫他留在家里。
③让步状语从句，although引导的句子可以转换成as的用法。
eg.Although she may fail....Fail as she may
eg.Although the graph is simple.Simple as the graph is.
eg.Although he is rich,he doesn't have any friends.Rich as he is,he doesn't have any friends.
④方式状语从句(就像....一样;正如....一样);
eg.Do as the instruction says.按照说明书上的要求去做。
He came as we expected.他就像我们所期待的那样来了。
⑤比较状语从句
eg.I'm as tall as you.
as短语：
as long as只要;such as例如;as soon as一....就......;so as to为了;as if好像;
as though好像;just as正如;
You can go where you like as long as you getback before dark.
你可以去你任何想去的地方，只要你在天黑之前回来。(as long as引导条件状语从句)He talks as if he knew all about it.
他谈起这件事来就好像什么都知道似的。(as if引导方式状语从句，从句中常用虚拟语气)
It is just as I told you.情况正如我跟你说的一样。(just as引导表语从句)
