四、用作形式主語
1. 基本用法
當不定式、動名詞、從句等用作主語時，避免頭重腳輕，通常在句首使用形式主語it，而把真正的主語放在句子末尾：
Is it necessary to tell his father everything? 有必要把這一切都告訴他父親嗎?
It’s no good sitting up too late. 熬夜沒有好處。
It is quite clear that he has read the book. 很顯然，他讀過這本書。
It doesn’t matter what he says. 他說什麼沒關係。
2. 幾種特殊的形式主語
(1) 用作動詞look, seem, appear, happen, occur, follow等的主語(後接that從句或as if從句)：
It seems as if it is going to rain. 看來要下雨了。
It seems that he knows everything. 他好像什麼都知道。
It (so) happened that I had no money with me. 碰巧我沒帶錢。
From this witness it follows that he must be guilty. 根據該證人的證詞判斷，他一定有罪。
(2) 當系表結構後接有if 或when引出的狀語從句時，通常在系表結構前使用形式主語表示說話人對某情況的看法：
It was a great surprise to me when he did a thing like that. 他這樣做使我很吃驚。
It would be a tragedy if anything went wrong. 如果出了什麼事就是一個悲劇。
(3) 用於其他固定句型，如if it were not for…(若不是因為)，it’s time that…(該…了)等。
3. 誤用形式主語的幾種情形
(1) 在該用it作形式主語的地方而誤用其他詞：
Is ______ necessary to tell his father everything?
A. it B. that C. what D. he
此題的正確答案應是A(it用作形式主語)，而不能選D。
(2) 在不該用形式主語的地方而誤用形式主語：
______ must be something wrong with the machine.
A. There B. It C. He D. That
此題應選A，為there be句型與情態動詞must連用的情形。
比較以下兩組句子：
______ is known to us all, the earth turns around the sun.
______ is known to us all that the earth turns around the sun.
A. It B. Which C. As D. That
前一句填C，as用以引導一個非限制性定語從句；後一句填A，it為形式主語，真正的主語是其後的that從句。
______ is difficult to persuade her to stay.
______ is difficult is to persuade her to stay.
A. It B. What C. He D. That
前一句填A，it為形式主語；後一句填B，what用以引導主語從句。注意後一句比前一句在不定式前多了個is。
五、用作形式賓語
1. 基本用法
當不定式、動名詞、從句等用作賓語且其後跟有賓語補足語時，就應在賓語補足語前使用形式賓語，而將真正的賓語移至句末：
We found it difficult to persuade her. 我們發現很難說服她。
He makes it a rule to get up before dawn. 他習慣於天亮前起床。
I think it very strange that he goes out walking almost every night. 我認為他幾乎每晚都出去散步是很奇怪的。
2. 幾種特殊的形式賓語
通常意義的形式賓語主要見於真正賓語為不定式、動名詞、從句等且其後跟有賓語補足語時的情形，即用於「動詞+it+賓語補足語+真正的賓語」這樣的結構，但以下兩類形式賓語卻比較特殊，其後沒有賓語補足語：
(1) 由於介詞後通常不能直接跟that從句作賓語，有個別特殊結構就在介詞後先接it作形式賓語，再跟that從句：
You may depend on it that he will turn up in time. 你可以相信他會及時趕到的。
I can’t answer for it that the boy is honest. 我不能保證這個男孩是誠實的。
I』ll see to it that all these letters will be sent to the post before twelve. 我將注意讓所有的信件在12點以前送到郵局。
【注】① 在其他情況下解決介詞後跟that從句的辦法是借用the fact作賓語：
They were worried over the fact that you were sick. 他們為你生病發愁。
② 個別介詞(主要是but和except)也可接that從句作賓語：
I know nothing about him except that he lives next door. 我只知道他住在隔壁，其他的就不知道了。
(2) 有的動詞由於通常只用作及物動詞，當它們後接if從句或when從句，通常應先在動詞後接it作形式賓語：
She won’t like it if you arrive late. 她不喜歡你遲到。
He hates it when people use his bike. 他討厭別人用他的自行車。
I hate it if you say such things in public. 我討厭你在大庭廣眾之下說那樣的事。
I』d prefer it if I didn’t have to do so much work. 要是我不必做那麼多工作，那就太好了。
I would appreciate it very much if you would help me with it. 如果你能幫助我做這事，我會十分感激。
【注】能這樣用的動詞不多，主要的有enjoy, like, dislike, love, hate, prefer, appreciate等表示情緒的動詞。而且高考對此也有所考查，請看一道高考真題(答案選A)：
I hate ______ when people talk with their mouths full.
A. it B. that C. these D. them
(3) 用於 have it that(說，認為)，take it that(認為)等個別固定表達中：
I take it (that) you agree. 我以為你同意了。
Report has it that about 30 people were killed in the accident. 據報導，在這次事故中大約有30人喪命。
六、用於強調句型
1. 基本句型結構
it用於強調句的基本句型結構是：It+be+被強調部分+that(who)+其他部分。請看用例：
It was in the Japan that he died. 他是死於日本。
It was yesterday that he got married. 他是昨天結婚的。
It was a computer that he bought last week. 他上個星期買的是一臺電腦。
2. 使用強調句型應注意的幾點
(1) 強調主語時，that後謂語動詞的數應與被強調的主語保持一致：
It is I who am right. 是我對。
It was you that were wrong. 是你錯了。
(2) 不要想當然地用he, she, that等代替強調句中的it：
_____ was in this room that he was killed.
A. It B. That C. He D. Who
此題答案應選A。該句還原成非強調句為He was killed in this room.
【注】有關強調句型的其他用法說明，參見本講義「強調句型」。
七、重點考點原創精練
1. Luckily, _____ happened to be a foreign teacher there.
A. it B. there C. they D. that
2. _____ is said to be no life on the moon.
A. It B. There C. That D. He
3. You should make _____ a rule to leave things where you can find them again.
A. it B. that C. you D. this
4. _____ is known to everybody, the moon travels round the earth once every month.
A. It B. As C. That D. What
5. I want very much to buy a color TV set, but I can’t afford _____.
A. it B. one C. this D. that
6. I should prefer _____ if you didn’t go there alone.
A. it B. that C. you D. which
7. He is always really rude, ______ is why people tend to avoid him.
A. that B. it C. this D. which
8. Having a trip abroad is certainly good for the old couple, but _____ remains to be seen whetherthey will enjoy it.
A. that B. it C. this D. which
9. Dear me! Just look at the time! I had no idea _____ was so late.
A. it B. as C. that D. what
10. _____ is hard is to do good all one’s life and never do anything bad.
A. He B. It C. That D. What
11. _____ is hard to do good all one’s life and never do anything bad.
A. He B. It C. That D. What
12. ____ doesn’t seem to have been much difficulty in solving the problem.
A. It B. That C. There D. He
13. I can’t remember a Christmas when _____ snowed so much.
A. it B. that C. what D. the weather
14. _____ was when she was about to go to bed that the telephone rang.
A. It B. That C. There D. He
15. Who was _____ that saved the drowning girl?
A. that B. it C. what D. he
【參考答案】1—5 BAABB 6—10 ADBAD 11—15 BCAAB
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（編輯：何瑩瑩）