Less or Fewer

词可不可数。如果是不可数名词，用less；如果是可数名词，用fewer。这个原则在语法考试的时候的确屡试不爽，但在实际应用中就站不住脚了。
“Less or fewer, that is a question.”
举个典型的例子：在美国的超市买东西，结账时可以选择走普通道或快速道。快速道上必定有显眼的标志：For X items or less（购买不超过X 件物品的顾客可使用此道）。到目前为止，我还没看到过任何一家超市用“For X items or fewer”。
被许多语言学家推崇的Merriam-Webster's Concise Dictionary of English Usage 有专门的less/fewer 条目，里面确认了这一现象：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. （通常的原则是：fewer形容可数名词，跟“数”挂钩；less形容不可数名词，跟“量”挂钩。这个原则简便易行，只有一个缺陷——它并不能准确描述所有的用法。）关键问题就在于， 什么时候会出现特例？
1 如果可数名词是距离、金额或时间单位，less than 比fewer than 更常用。
比如：Theodometer showed less than ten thousand miles.（里程表显示车开了不到1 万英里。）He had somewhat less than a million dollars to his name when he went to Washington. （刚去华盛顿的时候，他的财产还不到100 万美元。）
I've known you less than twenty-four hours. （我认识你还不到24 小时。）
2 如果出现“不少于某具体数字”的句式，no less than 比no fewer than 常用。
比如：This language is spoken by no less than 100 million people in the world. （全世界说这种语言的人不少于1亿。）
3 如果出现“某具体数字或更少”（也就是“不多于某具体数字”）的句式，一般用X or less，而非X or fewer。前面提到的超市快速道的X items or less 就属于这种情况。
再举一例：Readers are encouraged to keep their comments to 500 words or less. （读者最好将留言控制在500字以内。）
4 如果“少”跟在“一”后面，用one less，而非 one fewer。
比如：Every time you keep quiet and refuse to step up to the plate, you have one less chance to prove yourself. （每当你保持低调、不肯出面采取行动的时候，你就少了一次证明自己的机会。）
由此可见less 比fewer 要更灵活一些。前者同时适用于可数和不可数名词，而后者只适用于可数名词。
Merriam-Webster 的总结很精辟：
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. （如果英语是你的母语，用less 还是fewer，靠“听起来顺不顺耳”就可以判定。如果英语不是你的母语，“less对应不可数名词、fewer对应可数名词”的简单原则可作可靠引导，除了上面列出的特例。在这些情况下，less 是更好的选择。）