1. as用作關係代詞和關係副詞引導限制性定語從句，並在從句中作主語、賓語、狀語或表語，構成the same
as, suchas, soas, asas等結構，也就是說，先行詞之前一旦出現same, such, so, as這些詞時，而先行詞的後面又跟著一個從句，這時就要考慮是不是要用as作為關係詞來連線這個詞和這個句子。 例如：
No period in history has had as many important changes as have taken place in the past century. (as 作主語) It is so easy a book as every schoolboy can read. (as作賓語)
A wise man seldom talks about such things as he doesn’t understand.(as作賓語)
I shall do it in the same way as you did.(as作狀語)
Her skin is as white as snow is. (as作表語) 注意：(1)關係代詞as在限制性定語從句時，在從句裡做賓語時，絕不能省略。為什麼我要讓大家注意呢?
因為大家會聯想到that和which引導定語從句，做從句裡的賓語時可以省略。但是，大家要記住，我們今天講的是as引導的定語從句，as在定語從句裡做賓語時，是絕不能省略的。沒有為什麼，這就是語言的習慣。舉幾個例子大家看看。
Nowadays, women can receive the same pay as men can receive. 這句話裡面，as就絕不能省略。 而這句話
Nowadays, women can receive the same pay that men can receive. 裡面的that就可以省略。 所以，這句話也可以說成 Nowadays, women can receive the same pay men can receive.
(2)剛說過的這個例子大家可能已經注意到the same as 和the same that這兩個結構了。當先行
詞被 the same 所修飾時，關係詞既可以用 as，也可以用 that，那麼這兩者又有什麼區別呢?在表示具體事物時，有時兩者有一定的區別。一般說來，表示同一種類多用 as，表示同一事物多用 that，來看個例子：
This is the same instrument as I used yesterday. 這臺儀器跟我昨天用過的那臺一樣。 This is the same instrument that I used yesterday. 這就是我昨天用過的那臺儀器。 大家看這兩句話的意思已經明白它們的區別了吧?as表示的是一樣的，同一種類的事物，但會不
會就是那個他用過的.呢?不一定。而that則完完全全地是我用過的。
但是表示抽象概念時，同種類和同一事物是沒有絕對區別的，所以兩個詞可換用，請看例句 I have the same opinion as / that you have.
2. as引導非限制性定語從句。as在定語從句中作主語、表語或賓語。as代指整個主句，而as引導的這個定 語從句是用來補充說明整個主句的，可以放在主句之前或者之後。 例如：
As we all know, he studies very hard.(as代整個句子，作賓語) 也可說 He studies very hard, as we all know. As is known to all, he is the best student in our class.(as代整個句子，作主語) 也可說 He is the best student in our class, as is known to all. 一些常見的句式有：as is said above, 正如/就像上面所說的， as is already mentioned above, 正如/就像剛才
已經提到的，as is known to all, 正如/就像大家都知道的，as is reported in the newspaper, 正如報紙上報道的那樣。大家發現特點沒有啊?as在句子裡面的意思是“正如”“就像”
注意：(1)關係代詞which和as在引導非限制性定語從句時是有區別的，它們的區別就是是：which引導的非 限制性定語從句，絕不能放在句首，而as則可以;就像我們剛看到的例子一樣， He studies very hard, as we all know. 也可說As we all know, he studies very hard.
而要是換成which的話，這句話只能說成是 He studies very hard, which we all know. 在句中時，as有“正如”“就像”之意，而which則沒有此意。現在請大家看這兩句話 He studies very hard, which we all know. He studies very hard, as we all know.
請問這兩句話有什麼區別嗎?答案是有。儘管as和which都做know的賓語，但是我來看看這兩句的意思。He studies very hard, which we all know. 這句話意思是他學習很努力，這點我們都知道。而He studies very hard, as we all know. 這句話意思是他學習很努力，正如大家知道的那樣。大家體會出差別來沒有?大家做道題
Beijing government puts more than 700 million Yuan to increase its green space this year, ___ doubles the money provided last year. 這句話裡面填as還是which?把兩個詞帶到句子裡面分別體會一下意思。
(2)which引導非限制性定語從句作從句裡面的介詞賓語時，介詞可以提到which前面，請看例句
Eric received training in computer for one year, after which he found a job in a big company. 而as引導非限制性定語從句作從句裡面的賓語時，不能把介詞提到as前面的。
