Here is the rule as it is usually encountered: fewer refers to number among things that are counted, and less refers to quantity or amount among things that are measured. This rule is simple enough and easy enough to follow. It has only one fault—it is not accurate for all usage. If we were to write the rule from the observation of actual usage, it would be the same for fewer: fewer does refer to number among things that are counted. However, it would be different for less: less refers to quantity or amount among things that are measured and to number among things that are counted. Our amended rule describes the actual usage of the past thousand years or so.
As far as we have been able to discover, the received rule originated in 1770 as a comment on less:
This Word is most commonly used in speaking of a Number; where I should think Fewer would do better. No Fewer than a Hundred appears to me not only more elegant than No less than a Hundred, but more strictly proper —Baker 1770
Baker's remarks about fewer express clearly and modestly—"I should think," "appears to me"—his own taste and preference. It is instructive to compare Baker with one of the most recent college handbooks in our collection:
Fewer refers to quantities that can be counted individually.... Less is used for collective quantities that are not counted individually... and for abstract characteristics —Trimmer & McCrimmon 1988
Notice how Baker's preference has here been generalized and elevated to an absolute status, and his notice of contrary usage has been omitted. This approach is quite common in handbooks and schoolbooks; many pedagogues seem reluctant to share the often complicated facts about English with their students.
How Baker's opinion came to be an inviolable rule, we do not know. But we do know that many people believe it is such. Simon 1980, for instance, calls the "less than 50,000 words" he found in a book about Joseph Conrad a "whopping" error. And usage writers are not the only ones:
In Dunedin, Fla., Margaret Rice objects to the use of "less" when "fewer" is meant —James J. Kilpatrick, Hackensack(N.J.) Record, 28 June 1985
The OED shows that less has been used of countables since the time of King Alfred the Great—he used it that way in one of his own translations from Latin—more than a thousand years ago (in about 888). So essentially less has been used of countables in English for just about as long as there has been a written English language. After about 900 years Robert Baker opined that fewer might be more elegant and proper. Almost every usage writer since Baker has followed Baker's lead, and generations of English teachers have swelled the chorus. The result seems to be a fairly large number of people who now believe less used of countables to be wrong, though its standardness is easily demonstrated.
In present-day written usage, less is as likely as or more likely than fewer to appear in a few common constructions. One of the most frequent is the less than construction where less is a pronoun. The countables in this construction are often distances, sums of money, units of time, and statistical enumerations, which are often thought of as amounts rather than numbers. Some examples:
The odometer showed less than ten thousand miles —E. L. Doctorow, Loon Lake, 1979
... he had somewhat less than a million to his name when he went to Washington —David Halberstam, Harper's, February 1971
I was never in Europe for less than fourteen months at a time —James Thurber, letter, 18 July 1952
... the present enrollment of less than three thousand students —John Fischer, Harper's, February 1971
Her agency, less than 5 years old, is a smashing success —Donald Robinson, Ladies' Home Jour., January 1971
Begun with a capital of less than twenty pounds, it brought... financial security —Current Biography, December 1965
... an allied people, today less than 50,000 in number —W. B. Lockwood, A Panorama of Indo-European Languages, 1972
"... I've known you less than twenty-four hours...." —Agatha Christie, Why Didn't They Ask Evans?, 1934
Fewer can be used in the same constructions, but it appears less often than less. It is sometimes used in such a way as to make one suspect that an editor rather than a writer is responsible for the fewer.
... Dudek's car has fewer than 600 miles on the odometer —Rick Reilly, Sports Illustrated, 2 Dec. 1985
... has never gained fewer than 1,222 yards in a season —Rick Telander, Sports Illustrated, 5 Sept. 1984
From fewer than 15,000 in 1960, they reached 60,000 by 1970 —Norman Myers, International Wildlife, January/February 1982
Some contemporary usage writers (as Bernstein 1977, Chambers 1985, Cook 1987) concede that this use of few is acceptable.
The no less than construction noticed by Baker tends—over 200 years later—still to have less more often than fewer:
... about 26,000 acres worked by no less than 1,800 slaves —Times Literary Supp., 27 Aug. 1971
The class of 1974 ... included no less than 71 new Democrats —Tip O'Neill with William Novak, Man of the House, 1987
I can remember no less than five occasions when.... —Noel Gilroy Annan, ACLS Newsletter, January-February 1969
It is spoken by no less than 100 millions in Bengal and bordering areas —W. B. Lockwood, A Panorama of Indo-European Languages, 1972
Many of you have seen signs on the express lanes at supermarkets saying, "Twelve items or less"; and others, perhaps, may recall the contests in which a sentence was to be completed "in twenty-five words or less." Less is the choice in this construction:
... readers are encouraged to keep their comments to 500 words or less —Change, January-February 1971
... of all the millions of families in the country, two out of three consist of only three persons or less — Mark Abrams, London Calling, 9 Oct. 1952
... and now know enough to create little fictions that in 30 seconds or less get right to the heart of desire itself —Mark Crispin Miller, Johns Hopkins Mag., Winter 1984
Kilpatrick 1984 defends this less and the one just above. Less is again the choice in mathematical usage:
8 times 2 is less than 6 times 3 —Max A. Sabel et al., Mathematics, Book 1, 1977
In the geometries of Bolyai and Lobachevski... the sum of the angles of a triangle is always less than 180°—Robert W. Marks, The New Mathematics Dictionary and Handbook, 1964
Less is also frequent when it follows a number:
... almost $10 million less than for 1969 —Annual Report, Borg-Warner Corp., 1970
Many bulls fought in Madrid weigh 100 kilos less — Tex Maule, Sports Illustrated, 29 July 1968
... at thirty-three on my part, and few years less on yours —Lord Byron, letter, 17 Nov. 1821
And of course it follows one:
... one less scholarship —Les A. Schneider, letter to the editor, Change, September 1971
One less reporter —Don Cook, Saturday Rev., 24 June 1978
Less is also frequently used to modify ordinary plural count nouns. You will notice from the following examples that in present-day English this usage appears to be more common in speech (and reported speech) than it is in discursive writing. It must also be conceded that some of the plural nouns in the examples were probably thought of as uncountable amounts rather than numbers.
We have more and more wonderful means of saying things, and less and less wonderful things to say — Leslie Lieber, quoted in Einstein 1985
... Goldsmith took less pains than Pope... to create images of luxury in the reader's mind —John Butt, English Literature in the Mid-Eighteenth Century, edited & completed by Geoffrey Carnall, 1979
... Americans pay less taxes than most of the inhabitants of developed countries —Robert Lekachman, quoted in Center Mag., January-February 1970
... has considered advertising the fact that he has less commercials —Robert Lewis Shayon, Saturday Rev., 20 Aug. 1966
The less sodium you consume, the less drugs you're likely to need —Jane E. Brody, N. Y. Times, 11 July 1979
Less gallons means that they sometimes have to up their rates —Jack Cooper, quoted in N. Y. Times, 29 Oct. 1979
You have to make less mistakes —Victor Temkin, quoted in N. Y. Times, 4 May 1980
There are fewer industries and less job openings — Illustrated London News, 31 Aug. 1968
... lower rates ... lazy days, and less crowds —L. Dana Gatilin, Christian Science Monitor, 23 Oct. 1979
Less people exercise their right to vote —William Scranton, quoted in Celebrity, October 1976
When there are several distinct but equivalent mathematical proofs, that proof which has less steps and/ or utilizes less theorems is considered to be the more elegant —Nehemiah Jordan, Themes in Speculative Psychology, 1968
The examples above show native speakers and writers of English using less of count nouns in various constructions. Fewer could have been used in many of them— at times it might have been more elegant, as Robert Baker thought—but in others no native speaker would use anything but less. If you are a native speaker, your use of less and fewer can reliably be guided by your ear. If you are not a native speaker, you will find that the simple rule with which we started is a safe guide, except for the constructions for which we have shown less to be preferred.(资料出处：韦伯斯特英语用法词典)