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關閉窗口 （2）which作賓語時，有時可以替換為as。試對比：
●He is from the south, which we can know from his accent.（不能插入it）
●He is from the south, as we can know (it) from his accent.（有無it都可以）
（3）which作表語時，有時可以替換為as。試對比：
●He seemed a foreigner, which in fact he was.（不能插入a foreigner）
●He seemed a foreigner, as in fact he was (a foreigner).（有無a foreigner都可以。因為，在狀語從句中，承前省略了a foreigner。）
總之，as是連詞，which是代詞，兩者截然不同。二者能換用的情況，純屬巧合，并不能說明它們詞性相同。打個比方，一個馬群和一個牛群，偶爾在同一地方吃草，但是，馬還是馬牛還是牛。
3.常見的as引導評述性狀語從句。見下表，
|
分類 |
舉例 |
|
主系表 |
as (it) is often the case，as (it) is natural，as (it) seems likely |
|
主系+v-ed |
as (it) was pointed，as (it) is mentioned/repeated/said， as (it) have been announced，as (it) has been said before |
|
主謂 |
as (it) often happens，as (it) appears |
|
as I remember/understand (it)，as we had expected (it)， as I have said (it)，as we see/know (it)，as everyone knows (it) |
注：關于狀語從句中的省略，可以參看本章第七節以及第十二章。
4. 注意as/it/what的區別。如，
（what引導主語從句，that引導表語從句）
●It is known to us all that
（it作形式主語，真實主語是后邊的that從句）
●As is known to us all,
（as引導評述性狀語從句）
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