one
1.关于one和a的区别，参见a, an 20
下面两句里的a和an不可改作one：
- About a thousand people attended the lecture.
- We spent nearly an hour in the park.
2.下面两句里都有a：
- This book is a good one.
- This book is such a one as is needed by every student of English.
但下面两句里都没有a：
- He has many books, I want to borrow one.
- This book is one that is needed by every student of English.
你也许要问是否可以说This book is the one that is needed by every student of English。有the和没有the全句的意思不同：没有the，全句的意思是“这是一本每一个研究英语的人所需要的书”。有了the，全句的意思是“这便是那每一个研究英语的人所需要的（唯一的）书”。
可以说a good one，也可以说one good one。
3.不可说my one(s), your one(s), his one(s)等，也不可说John's one(s), Smith's one(s), John Smith's one(s)等，如不可说：
- Your cat is black, my one is white.
- All his friends are artists, all her ones are teachers.
- My house is larger than John's one.
- His books are kept downstairs, and his brother's ones are kept upstairs.
第一句里的my one该改作mine，第二句里的her ones该改作hers，第三句里的one和第四句里的ones都该删去。
但假使在那one或ones前面加上了形容词，那one或ones便用得不错，而且不可省去。例如：
- Your cat is black, my young one is white.
- All his friends are artists, all her new ones are teachers.
- My house is larger than John's red one.
- His books are kept downstairs, and his brother's English ones are kept upstairs.
4.these ones和those ones很不普通，最好不说，把ones删去：
- These ones are too small. I want those ones.
但假使在ones前面加上了形容词，那ones便不可省去。例如：
- These green ones are too small. I want those black ones.
5.注意下面两句里用have和are，不用has和is：
- This is one of the best books that have ever been written on the subject.
- She is one of the few girls who are acquainted with the old poet.
第一句里的that指books，不指one，所以该用have，第二句里的who指girls，不指one，所以该用are。但注意下面两句里用has和is，不用have和are：
- This is the one of the books on the subject that has ever been written in Chinese.
- She is the only one of the few girls of my acquaintance that is acquainted with the old poet.
第一句里的that指the one，不指books，所以用has，第二句里的that指the only one，不指girls，所以用is。
6.注意下面各种表达方式的结构：
- one of the best, if not the best, of books,
- one of the best books, if not the best,
- one of the, if not the, best of books,
- one of the, if not the, best books,
- one of the, if not the, best book,
- one of the best, if not the best, books,
- one of the best, if not the best, book
第一种表达方式和第二种表达方式不错。第三种表达方式不对，因为best兼指单数和复数。第四种表达方式不对，因为if not the只能等于if not the best books，不能等于if not the best book。第五种表达方式不对，因为one of the best book当然不通。第六种表达方式不对，因为if not the best只能等于if not the best books，不能等于if not the best book。第七种表达方式不对，因为one of the best book当然不通。
7.注意下面各句里的斜体部分：
- One should take care of one's health.
- One should take care of his health.
- One of them does not take care of his health.
- One of them does not take care of her health.
- One who does not take care of his health is likely to be ill at any time.
- He behaves like one who never takes care of his health.
- She behaves like one who never takes care of her health.
- One has to say here that one always takes care of one's health.
第一句和第二句意思相同，One都指“任何人”。第一句里用one's是英国用法，第二句里用his是美国用法。但这种“美国用法”并不限于美国，在苏格兰也很普通，在二三百年前，在英国也很普通，现在也还有许多英国人用。有下面四点该注意：
（1）既然用one's指“任何人”的one，同样地也用one和oneself。例如：One must occasionally do things that one dislikes.和One should learn to express oneself clearly.
（2）在one指“任何人”的时候，不可用they, them, their或themselves指它，如不可说One should take care of their health。
（3）不可用one, one's或oneself指anyone, everyone, noone或someone，如不可说Everyone should take care of one's health，该把one's 改作his。
（4）句子里多用了one和one's往往不自然：如One knows one's friends will help one if one is in trouble, or at least one hopes one's friends will help one嫌不自然。
第三句里用his，因为指男子。第四句里用her，因为指女子。one of them的是男是女或者非男非女当然一定，one并非指“任何人”，his或her或者its决不可改作one's。假使one of them里的them包括男和女，说话者又不能确定那one of them是男是女，那末该用his or her。例如：One of them does not take care of his or her health.
第五句里的One虽然也指“任何人”，但因为后面有who...这一从句，习惯用he, him和himself指它。
第六句里用his，因为指男子。第七句里用her，因为指女子。
第八句实际等于I have to say here that I always take care of my health。这种用法有人以为是不正当的。下面各句里都有这用法：
- One is rather busy just now.
- One is glad to have done it.
- He told me so, and one was very glad.
- One regards him as one's best friend.
8.one or two被当作复数形式。例如：There are one or two things that I have to mention. one or two通常作“若干”解，很少作“一或二”解。
9.关于one...the other...和the one...the other...，下面几点该注意：
（1）说“一个”和“另一个”而不说明“前后”通常用one...and the other ...。例如：I have two brothers, one is fifteen and the other is eleven.但也有人在one前面加上the。
（2）说明“前后”必须用the one...the other...。例如：I have two brothers, Paul and Richard, the one（指Paul）is fifteen and the other（指Richard）is eleven.
（3）在（2）里看来，the one指“前者”，the other指“后者”，这是现代英语里的用法。从前却把the one指“后者”，the other指“前者”。
