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 allaknow， as it is know， as is know 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 it 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）
Boy as he was， he was chosen king.
他虽是孩子，却被选为国王。（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)
