由as, which 引導(dǎo)的非限定性定語從句,as和which可代整個(gè)主句,相當(dāng)于and this或and that。As一般放在句首,which在句中。例如:
As we know, smoking is harmful to one's health. 如我們所知,吸煙有害健康。
The sun heats the earth, which is very important to us. 太陽使地球暖起來,這對(duì)我們?nèi)祟惡苤匾?/span>
典型例題
1)Alice received an invitation from her boss, ___came as a surprise.
A. it B. that C. which D. he
答案C. 此為非限定性從句,不能用 that修飾,而用which. it 和he 都使后句成為句子,兩個(gè)獨(dú)立的句子不能單以逗號(hào)連接。況且選he句意不通。
2)The weather turned out to be very good, ___ was more than we could expect.
A. what B. which C. that D. it
答案B。which可代替句子,用于非限定性定語從句,而what不可。That 不能用于非限定性定語從句,it不為連詞,使由逗號(hào)連接的兩個(gè)句子并在一起在英語語法上行不通。
3)It rained hard yesterday, ____ prevented me from going to the park..
A. that B. which C. as D. it
答案B. as 和which在引導(dǎo)非限制性定語從句時(shí),這兩個(gè)關(guān)系代詞都指主句所表達(dá)的整個(gè)意思,且在定語從句中都可以作主語和賓語。但不同之處主要有兩點(diǎn):
(1)as 引導(dǎo)的定語從句可置于句首,而which不可。
(2)as 代表前面的整個(gè)主句并在從句中作主語時(shí),從句中的謂語必須是系動(dòng)詞;若為行為動(dòng)詞,則從句中的關(guān)系代詞只能用which.。在本題中,prevent由于是行為動(dòng)詞,所以正確選項(xiàng)應(yīng)為B。
as 的用法
例1. the same…as;such…as 中的as 是一種固定結(jié)構(gòu), 和……一樣……。例如:
I have got into the same trouble as he (has). 我碰到了跟他一樣的麻煩。
例2. as可引導(dǎo)非限制性從句,常帶有'正如'。例如:
As we know, smoking is harmful to one's health. 如我們所知,吸煙有害健康。
As is known, smoking is harmful to one's health.
As是關(guān)系代詞。例1中的as作know的賓語;例2中,它充當(dāng)從句的主語,謂語動(dòng)詞know要用被動(dòng)式。