More and more, fewer people use “less” and “fewer” the way the language gods intended. “There are less people here than there were last year,” for example, is commonly heard or written.
Grammar texts are pretty absolute: Use “fewer” when you’re talking about countable things, and “less” when you’re not. So that first example should be “fewer” people, because you can count them. Simple.
OK, then what about this? “The contract pays him fewer than a thousand dollars a week.” Doesn’t that sound funny? But it must be right: You can count those dollars.
Thus the first “exception” to the “rules”: Use “less” when the context refers to a quantity, rather than individual things, even if there are a number of things. Thus, “he earns less than a thousand dollars a week,” but “he earns five hundred fewer dollars than his wife.”
That “exception” is usually easy to follow: Use “less” with singular nouns and “fewer” with plural nouns. (You need “less” salt on your fries, but “fewer” grains of salt.) If you’re comparing quantities of individual things, “fewer” usually works better than “less.” (You earn “less” money but “fewer” dollars than someone else.)
Even so, there are times when you can’t tell if you’re dealing with individuals or groups. In 2008, The New Yorker ran this cartoon of a checkout lane with the sign “10 items or less“ corrected to “10 items or fewer.” After all, you can count each of those items, yes? Except that almost no one says “ten items or fewer.” Maybe it’s one grocery order, so the individual items don’t count? Hey! You have eleven items! Put one of them back, so you have one item fewer.
Except that you should say “one item less.”
Other “exceptions” include distances (“less” than ten miles away), percentages and fractions (“less” than two-thirds of the voters), time (“less” than sixty seconds), measurement (“less” than thirty square yards), etc., etc., etc. (And, by the way, you shouldn’t use “fewer number,” as in “a fewer number of people.” When you say “fewer,” you’re already signaling that you’re talking numbers.)
Lesser mortals have failed to keep those exceptions straight. And Merriam Webster’s Dictionary of English Usage points out that the “rule” actually started out as a “guideline,” where common sense, ear and elegance trump “right” and “wrong.”
Perhaps you can think of it as Garner’s Modern American Usage describes it: “Fewer emphasizes number, and less emphasizes degree or quantity.”
Yes, it requires you to think, but there are less fruitful ways to waste a day.