5. It's polite to finish eating everything on your plate, so don't take more food than you need. 餐盘里的东西要吃光才礼貌，所以不要多拿。
该句中的it为形式主语，真正的主语为to finish eating …例如：It's not right to tell lies. 撒谎是不对的。
it作为形式主语，真正的主语是动词不定式短语。常见的句型有：
4. Having good table manners means knowing, for example, how to use knives and forks, when to drink a toast and how to behave at the table. 在餐桌上，懂礼节意味着你知道如何使用刀叉，何时祝酒以及如何在用餐时举止得体。
该句中having good manners为v-ing形式作主语。例如：Collecting stamps is one of his hobbies.
▲友情提示：在v -ing 形式前加形容词性物主代词或名词所有格，构成v -ing复合结构，在句中作主语、宾语。
[考例5](2001上海)Fishing is his favorite hobby, and ____.
A. he'd like to collect coins as well
B. he feels like collecting coins, too
C. to collect coins is also his hobby
D. collecting coins gives him great pleasure
[考查目标]v -ing 作主语。
[答案与解析]D A、B、C三项句法都无错误，但在and连接的并列句中，两个简单句的主语要保持一致的形式 fishing and collecting coins分别做两个简单句的主语。
3. When asked about the secret of his success, Steven Spielberg said that he owes much of his success and happiness to his wife and children. 当有人问起他成功的秘诀时。史蒂文·斯皮尔伯格说起他的成功和幸福主要来自于妻子和孩子。
该句中的 "when" 是时间状语从句的省略形式。在状语从句中，如果从句主语与主句主语一致或从句主语是it，而且从句谓语动词是be或包含be时，常常将从句主语与be省略。例如：Although born in Chicago, the author is famous for his stories about New York.
[考例] (2003上海春招) Unless ___to speak，you should remain silent at the conference.
A. invited B. inviting
C. being invited D. having invited
[考查目标]状语从句的省略现象。
[答案与解析]A unless为连词，后面省略了you are，所以选invited。
2. Many people who saw the film were afraid to swim in the sea when they remembered the scenes in which people were eaten by the shark. 好多看过这个片子的人一想起片中鲨鱼食人的场面．就不敢下海游泳了。
该句是一个复杂长句，从when到句子末尾是状语从句，在从句中包含一个由which引导的定语从句，修饰先行词scenes; 在前面的主句里面。包含一个由who引导的定语从句，修饰先行词people。例如：Those who want to go camping next Sunday sign your name here before class is over.
定语从句关系词的选择，要遵循“瞻前顾后”的原则，所谓“瞻前”即看前面的先行词指人还是指物；“顾后”即后面的定语从句，看关系词在定语从句中作什么成分。例如：This is the factory where he works. (状语) / This is the factory (that / which) he visited. (宾语)
[考例](NMET 1992)In the dark street，there wasn't a single person ____ she could turn for help.
A. that B. who C. from whom D. to whom
[考查目标]定语从句。
[答案与解析]D “turn to sb for help”为固定短语，意思是“向某人求助”，所以选to whom。
2. why在句中是关系副词，引导定语从句，修饰先行词reason，同时它在定语从句中作状语，此时why = for which，但要注意：关系词在定语从句中作主语或宾语时，要用关系代词that或which。
[考例](2002上海春招)Is this the reason ____ at the meeting for his carelessness in his work?
A. he explained B. what he explained
C. how he explained D. why he explained
[答案与解析]A what，how不能引导定语从句，排除B、C两项；the reason在定语从句中作explained的宾语，可填that / which，或者也可以省略。
1. 句中that引导的表语从句说明主语reason的具体内容，往往被看作是固定句型：The reason is / was that clause. 当主语是reason / cause时，一般不能用because或why引导表语从句，以免造成语意重复。当主语是This / That时，可以由because / why引导表语从句。例如：One reason is that people traveled to America from all European countries.
[考例](NMET 1999)
-- I drove to Zhuhai for the air show last week.
-- Is that ____ you had a few days off?
A. why B. when C. what D. where
[考查目标] 表语从句。
[答案与解析]A 句子的意思是“那就是你请了几天假的原因吗?”因此可知答案为why。
1. The reason why he could not go there was that his grades were too low. 他没能上电影学院是因为他的分数太低了。
该句巾的why引导一个定语从句，而that引导表语从句。
5. If you had gone ____ your test paper carefully before handing it in you would have made fewer mistakes.
☆句型诠释☆
4. -- What do you think the contest? -- I was told that the English Speech Contest went ____ successfully last night.
3. The market was filled ____ salted fish, giving the worst smell that you can imagine.