第1个回答 2012-03-11
补充一点，than也可引导定语从句，一般做从句的主语（也可以作宾语）。
Families have also experienced changes these years. More families consist of one-parent households or two working parents; consequently, children are likely to have less supervision 【at home】 than 【was common in the traditional family structure】.
than 在从从句中充当主语，后面接谓语动词was，它指代的先行词名词是supervision。
There ought to be less anxiety 【over the perceived risk of getting cancer】 than exists in the public mind today.
Than就是指代anxiety在从句中做主语，因为anxiety是单数，所以谓语动词要用exists。【over the perceived risk of getting cancer】其修饰作用的，不要要被其蒙蔽啊。
These proposals sought to place greater restrictions 【on the use and copying of digital information 】than exist in traditional media.
先行词是restrictions，它是复数形式，所以，谓语动词要用exist。
The project requires more labor than has been put in 。
这里的than从句的主语，指代的先行词是labor，所以位于要用单数has。
Than引导定语从句的用法特点。
1 than可以做关系代词引导定语从句，在从句中一般做主语（也可以作宾语）.
2 than 前面的主句需要有形容词的比较级形式.
3 比较级所修饰的名词就是先行词。
第2个回答 2012-03-09
一、基本概念
（一） 定语从句
在复合句中作定语，修饰名词或代词的从句叫定语从句。
（二） 先行词
被定语从句所修饰的名词或代词。一般情况下，定语从句紧跟先行词。但也有因各种原因定语从句与先行词被分割的现象。
（三）关系代词和关系副词
定语从句的引导词。与先行词关系密切，因此紧跟先行词，并在定语从句中充当一个成分。关系代词有：who, whom, which, that和whose, 另外，as也可充当关系代词。关系代词在定语从句中充当主语、宾语、表语和定语。关系副词有：when, where和why。在定语从句中充当状语。
① Do you know the professor who/that will give us a speech next week? （作主语）
② I read a report about his new novel that/ which will soon be published. （作主语）
③ The plan that/which they argued about was settled at last. （作宾语）
④ This is the new secretary (who/whom/that) I would like to introduce to you. （作宾语）
⑤ The soldier whose legs were badly wounded was operated on without delay. （作定语）
注意：关系代词在定语从句中充当宾语时可以省略，充当主语时则不能。（见上例③④）
（二）关系代词that代替which的一些情况
which, that 在代替物时，一般可以通用。
但在有些情况下，只用 that。
⑴ 先行词是最高级形容词或它的前面有最高级形容词修饰时。
例如：
① This is the best that has been used against pollution.
② English is the most difficult subject that you will learn during these years.
⑵ 先行词是序数词，或它前面有一个序数词时。
例如：
① This is the last place (that) I want to visit.
② It is the first American movie of this kind that I’ve ever seen.
⑶ 先行词是 all, much, little, something, everything, anything, nothing, none等代词时。
例如：
① You should hand in all that you have.
② We haven’t got much that we can offer you.
⑷ 先行词前面有the only, the very, any, few, little, no, all, much, every等修饰时。
例如：
① The only thing that we can do is to give you some money.
② The little money (that) he had was stolen.
（三）宜用who, 而不用that的一些情况
⑴ 先行词是one, ones, anyone时。
例如：
① One who has nothing to fear for oneself dares to tell the truth.
② Don’t tell anyone about the news who oughtn’t to know it.
⑵先行词是those时。
例如：
① Those who were not fit for their work could not see the beautiful clothes made of the magic cloth.
（四）其它情况
⑴ 先行词既有人又有物时。
例如：
① Do you know the things and persons that they are talking about?
② The bike and its rider that had run over an old man were taken to the police station.
⑵ 主句已有疑问词who 或which时。
例如：
① Which is the bike that you lost?
② Who is the woman that was praised at the meeting?
（五）与whose有关的问题
⑴ whose是代词的所有格，它既可以代人也可以代物。
例如：
① I saw a woman whose bag was stolen.
② Please show me the book whose cover is red.
⑵ 当whose表示物与物的所有格关系时，亦可用of which的形式。
例如：
① The building whose roof you can see from here is a new restaurant.
→ The building, the roof of which you can see from here, is a new restaurant.或
→ The building, of which the roof you can see from here is a new restaurant.
三、介词前提的问题
关系代词在定语从句中充当介词宾语时，介词可以前提至关系代词前。
例如：
Have you seen the pen (which) I wrote the note with just now? （which作介词with的宾语）
→Have you seen the pen with which I wrote the note just now?
但是，要注意的是：
⑴ 介词前提后，先行词是人或物，关系代词分别只能用whom和which，而不再用that或who。
⑵ 介词前提后，关系代词不再能省略。
⑶ 有些含有介词的短语动词中的介词不能前提，如：look for, look after, take care of等。
例如：
① 错误：Who is the old man to that you were talking to?
正确：Who is the old man to whom you were talking ? 或 Who is the old man (that/ whom) you were talking to?
② 错误：These are the sheep of which the boy took care.
正确：These are the sheep (which/that) the boy took care of.
四、关系代词在从句中作主语时，从句中的主谓一致
关系代词在定语从句中作主语时，从句的谓语动词在人称和数上应与先行词一致。
例如：
① Who is the guy that is reading over there?
② The number of people that are going to the exhibition is expected to be over 25,000.
③ All that needs to be done has been done.
④ He is one of the students who use computer a lot for study.
⑤ Timmy is the only one of the pupils that has failed the exam.
例③中的all意为“一切”，作单数。例⑤中没通过考试的学生事实上只有一人，因此谓语动词也用单数。