不少读者来信问到关系代词that和which的区别。这的确是一个常使中国学生感到困惑的问题，也是一个久已有的老问题。读者来信只问到that和which指物时的区别，所以我们的讨论也只限于指物，不拟涉及指人的问题。
有的语法家认为用that或which指物是一个文体问题，应由说话人或写话人的风格而定。但也有不少语法家认为这不完全是个文体上的问题，二者实际上还是有区别的。总的说来，他们认为that多用在非正式英语中，which多用在正式英语中。但也有语法家认为that最普通，既可用在非正式英语中，亦可用在正式英语中。此外，有的语法家认为用that时，其先行词多不具体指一确定的事物，而且与先行词的关系比较密切，而which与其先行词的关系则比较松驰。
具体地讲，很多语法家认为that与which有下列一些不同： that多用于：
1. all, everything, something, anything, nothing, little, few, much等不定代词为先行词时，如：
(1)All that glitters is not gold.
(2)Nearly all the wine had been drunk, but we finished the little that was left.
(3)His writings contain much that is old and even trite.
(4)That hospital is one of a few that have benefited from the help of a United Nations population fund programme.
其他如everything, something, anything和nothing后接that的情况，比较常见，这里就不举例了。值得注意的是间接疑问句中的what用作先行词时，其后的关系代词须用that，如：
(5)I don't know what I can say that I've not said a thousand times already.
(6)What does she do that 's so different?
2. 先行词为形容词最高级所修饰时，如：
(7)It was the largest map that I ever saw.
3. 先行词为序数词以及only等所修饰时，如：
(8)The first thing that we should do is to work out a plan.
(9)That's the last thing that I want to do.
(10)These are the only things that matter.
4. 用在It is...that...分裂句（即强调结构）中，如：
(11)It was liberation that brought about a complete change in his life.
（严格说来，that在此是一种缩合连接代词，=that which）
但请注意在分裂句中，亦可用which代替that。有时甚至用which较好，因为用that会给人一种句子尚未完成的感觉，如：
(12)It is not only pictures which are beautiful.
（意思是“不仅图画是美丽的”。如用that，意思会变成“不仅是美丽的图画”这样未完成的句子）
5. 先行词为指示代词same与such修饰时，如：
(13)She was wearing the same dress that she'd on the day before.
(14)Such an act that blurs the grace and blush of modesty...
（此句摘自莎士比亚名剧《哈姆雷特》）
以上所谈的that用法大都可在一般语法书中见到。下面让我们列举一些一般语法书较少讲到的情况：
6. 先行词简短时，如：
(15)I'll take you to the building that all elderly university teachers prefer. （此句用that亦与口语化有关）
7. 先行词不具体特指某（些）事物时，如：
(16)Each made a list of books that had influenced him.
8. 从句中的主语为简短的人称时，如：
(17)Who took the dictionary that I bought yesterday?
9. 从句的主语较为复杂时，如：
(18)She tries to correct some misconceptions that politically romantic or conservative Americans express about China's modern history.
10. 定语从句表先行词的固有特点时，如：
(19)There is a house that has big windows.
（试比较the house which we have just passed）
11. 先行词表品质、地位等时，如：
(20)He has a look that is intellectual, but somewhat bloodless.
（先行词表品质）
(21)He hasn't the position that Brown has. （先行词表地位）
12. 关系代词用作表语时，如：
(22)It has taken us several decades to make it the beautiful city that it is today.
(23)The distance that you are from home is immaterial.
13. that=in which, for which等时，如：
(24)I wish you would see things in the light that we see them.
(that=in which, which代表the light)
(25)He took him for his model for the very reason that he ought to have shunned his example （that=for which, which代表the reason）
14. 具有如下的并列谓语的定语从句：
(26)The life-work that Acton collected innumerable materials for, but never wrote, was a History of Liberty.
（that...for显然较for which好）
15. 句中已有which需要有所变化时，如：
(27)Which was the hotel that was recommended to you?
(28)He enjoyed a lucrative practice which enabled him to educate his family with all the advantages that money can give.
(29)She cannot regain control of the threads of culture that she has let drop, which now lie in muddled tangles at her feet.
不消说，上述情况中大都也可使用which，只不过that更为常见而已。现将多用which的情况列举如下：
1. 关系代词离先行词较远时，如：
(30)Larry told her the story of the young airman which I narrated at the beginning of this book. （注意此句的先行词story与关系代词which之间有另一名词airman，在这种情况下一般皆用关系代词which）
2. 定语从句表补充重要情况时，如：
(31)Was I counting on Israel to work some miracle which would give me the strength?
3. “those+复形名词”之后的关系代词多用which，如：
(32)A shop should keep a stock of those goods which sell best.
4. 两个并列的定语从句中第一个关系代词用that时，如：
(33)I have eaten the plums that were in the icebox and which you were probably saving for breakfast. （但皆用that的情况似乎也不少）
(34)Luxury means something that is not necessary and not often had or done but which is very pleasant.
5. 先行词that之后，如：
(35)I have that which you gave me.
（that which比较正式，在非正式英语中常为what所代替）
6. 先行词之前有指示代词that用作定语时，关系代词亦常用which，如：
(36)That pen which he took is mine.
7. 非限制性定语从句的关系代词多用which（有的语法家认为必须用which，决不可用that，但事实上也有用that的情况）
(37)Beijing, which was China's capital for more than 800 years, is rich in cultural and historic relics.
8. 与用作定语的单词或短语并列的定语从句应用关系代词which，如：
(38)She has wanted to transfer to a job nearer to her home, but which still allows her to work in her field of electrical engineering.
（which从句与nearer to her home并列。但有的语法家认为这种用法是不规范的）
9. 介词之后须用which，如：
(39)This is the one of which I'm speaking.
（语法家们说关系代词that原本是一限定词，故其前不可用介词）
最后，还应指出，that与which用作宾语时，在口语中，尤其当从句比较简短时，常可省去不用，如：
(40)What a good harvest you've got this year!
（省去了及物动词get的宾语）
(41)That's the book he's been looking for. （省去了介词for的宾语）
再如上述例句（7）、（8）、（9）、（17）中用作宾语的关系代词皆可省去。
由于同样的原因，尤其在以there和it引导的句子中，用作主语的关系代词亦可省去，如：
(42)There isn't much could be done.
(much之后省去了用作主语的关系代词that)
(43)Here's something goes to the plane.
(something之后省去了用作主语的关系代词that)
(44)It is distance lends enchantment to the view.
(distance之后省去了用作主语的关系代词that)
(45)I've some news may cheer him.
(news后省去了用作主语的关系代词that)
比较正式的文件中有时亦可省去用作主语的that或which，尤其在含有插入语的定语从句（其实应唤作复合定语从句）中，如：
(46)That night a sleeping George lay in a sweat of anticipation over the punishment he knew was coming.
(punishment之后省去了用作主语的关系代词that或which，从句中含有摘入语he knew)
在非正式文体中，亦可省去用作表语的关系代词that，如：
(47)My typewriter is not the machine （that）it was.
(48)The house was large, with beautiful furniture. It was everything（that）ours wasn't.
其实以上所谓省去关系代词的用法古已有之，并非省略。
关于关系代词that与which指物时二者的区别，就谈这么多吧。
