记得学习as非限定性定语从句
导语：定语从句又分为限制性与非限制性定语从句，下面讲解as非限定性定语从句，欢迎参考。
关系代词 as 用于引导非限制性定语从句时，应注意以下三点：
一. as引导的非限制性定语从句位于句末时，一般可以和which互换。
The author was brought up in a small village, as (which) is recounted in some of his stories.
作者是在一个小村庄里长大的，这个问题在他的一些小说了就被提到过。
She is extremely popular among the students, as (which) is common knowledge.
她在学生中间很受欢迎，这是普遍知道的。
但是，当主句是否定句时，as和which就不能互换，因为as引导的非限制性定语从句不包含否定意义。
Spiders are not insects, as many people think.
蜘蛛不是昆虫，而许多人却认为蜘蛛是昆虫。
二. as引导的非限制性定语从句可以放在句子的最前面，而which引导的定语从句不能放在句首。
As is often the case, the girl forgot to her dictionary.（不能用which）
经常是这样子的，那个女孩忘了带字典。
As you will find out, all is now settled.
你将会看到这样的情况，一切都已搞定了。
三. as引导的非限制性定语从句可以放在句子的中间，which引导的定语从句也可以放在句子的中间。
Water, which is a clear liquid, has many uses.
水是一种透明的液体，有许多用途。
Taiwan is, as you know, an inseparable part to China.
众所周知，台湾是中国不可分割的一部分。
拓展
（一）非限定性定语从句as引导
as引出非限定性定语从句时，代替整个主句，对其进行说明但通常用于像as we all know, as it is known, as is known to all, as it is, as is said above, as always mentioned above, as is usual, as is often the case, as is reported in the newspaper等句式中。as在非限定性定语从句中作主语、表语或宾语，且引出的从句位置比较灵活，可位于句首或句末，也可置于主句中间。通常均由逗号将其与主句隔开。as有“正如……， 就像……”之意。
如：As is known to the United States, Mark Twain is a great American writer.
美国人都知道，马克·吐温是一位伟大的美国作家。（as在从句中作主语）
He forgot to bring his pen with him, as was often the case.
他忘了带笔，这是常事。（as在从句中作主语）
He is absorbed in work, as he often was.
他正在全神贯注地工作，他过去经常这样。（as在从句中作表语）
As we all know, the earth is round.
众所周知，地球是圆的。（as在从句中作宾语）
The two brothers were satisfied with this decision，as was agreed beforehand.
两兄弟对此决定都满意，这项决定在事前都已得到他们的同意。(as在从句中作主语)
Taiwan is，as you know，an inseparable part of China.
你知道，台湾是中国不可分割的一部分。(as在从句中作宾语)
“介词+关系代词”引导非限制性定语从句：在介词后引导非限制性定语从句。
难：句意相同
as it is known to all (that...) (as 做状语从句连词，是状语从句, 可以加that)
与 as is known to all (as做主语，是定语从句)
与 it is known to all that... (主语从句，it是形式主语)
即：As is widely known, the moon is closer to the us than the sun.(定从)
= That the moon is closer to the us than the sun is widely known.(主从)
= It is widely known that the moon is closer to the us than the sun.(主从)
As it is known to everyone, I thought you knew about it too.(状从)[1]
关系代词which有时并不代表主句中某一确定的词，而是概括整个主句的意思。介词的`选择取决于它与先行词的搭配或与从句中谓语动词的搭配。
They were short of sticks to make frames for the climbing vines，without which the yield would be halved.
他们缺搭葡萄架的杆儿，没有它们产量会减少一半。
They thanked Tom，without whose support they would not have succeeded.
他们很感激Tom，没有他的支持他们是不会成功的。
（二）as, which 非限定性定语从句
由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that，as, which 非限定性定语从句。As一般放在句首，which在句中。
As we know, smoking is harmful to one's health.
The sun heats the earth, which is very important to us.
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