單獨使用as時,as亦可作關係代名詞，等於

which，用以代替整個主要子句。

(例1)

He is nice, which is known to us.

= He is nice, as is known to us.   which/as代替He is nice

(例2)

He works hard, which we all know.

= He works hard, as we all know.  which/as代替He works hard

as引導的子句可置於主要子句前後，而which引

導的子句只能置於主要子句之後。

(例3)

He is nice, which we all know.

= He is nice, as we all know.

= As we all know, he is nice.

Which we all know, he is nice     ( x ).

as+be+p.p.時be動詞可以省略

例:He is nice, which was mentioned before.

=He is nice, as was mentioned before.

=He is nice, as mentioned before.

特殊例子:

He was drunk,as usual.

=He was drunk,as/which is usual with him.

例:His conditions are as follows.

=His conditions are as what follows.

例:He listed the names,as follows. (不管前面名詞單複數,動詞時態,as後面動詞一定要用follows))

(X)資料來源：長春藤 英文文法 賴世雄編著 p129~130

 He saw the girl, which delighted him. 
He saw the girl, as delighted him.    ... (X)

He admires Mrs. Brown, which surprises me very much.

He admires Mrs. Brown, as surprises me very much.    ... (X)

當as引導非限定從屬子句時，as代表了主句或主句一部分所表達的意思（不能是某個名詞或者代詞），在從從屬子句中作主詞。

例如：as is known to all等常見形式，非限定從屬子句是be動詞或者連綴動詞(指狀態)時，可用as或which做主詞，

(如)He married her, as/which was/seemed natural.都是正確的。 

非限定從屬子句是一般動詞(指動作)時，只能用which ，

(如) He married her, which delighted him.

 而He married her, as delighted him.卻是錯的。

delight為一般動詞(指動作)不能用as取代，

as有【正如、就像】的意思，which就沒有這樣的意思了。