This is an issue that has plagued people for ages. Well, people who know that it is a hotly debated topic, anyway! It’s difficult to remember what is recommended and when it is appropriate or allowable to use “less” instead of “fewer.” I did some research on this a while back and wrote an essay on it. Hopefully this clears things up a little! I now automatically pay special attention to the terms when I see or hear them, but I still have trouble remembering when to make exceptions, so I tend to be more forgiving.
The usage difference between “less” and “fewer” is clear once the words are defined, but it seems that for native speakers, the word “fewer” has fallen out of colloquial speech. It is common to use the word “less” when describing an amount, regardless of the number of the things being described. For example, it is not unusual to hear “less than three minutes” or “that class has less students” just as often as phrases like “use less salt” and “there is less oxygen up here.” The difference had not even occurred to me until a few years ago, but once I heard it, it was impossible to ignore the misuse.
The Cambridge Guide to English Usage says that “less” is considered a determiner (and sometimes a pronoun), used to mean “smaller in amount.” It uses examples with uncountable nouns (“beef” and “exposure”). According to the Guide, this particular use of the word isn’t up for debate, but its use as “fewer in number” certainly is, because of its prevalence in informal speech. The Guide goes into further detail in the “fewer or less” section. “Fewer” must be used with count nouns (nouns that you can pluralise) while “less” must be used with only collective (“family” or “class”) or mass nouns (“sugar” or “milk”). (Interestingly, they note that “more” can be used with both count and mass nouns.) The Guide also makes the observation that in written works, “fewer” is more prominent (instead of “less”) than it might be in spoken language, although “less” is still more common. This is true of many frequent mix-ups. No one is grammar-checking our speech, and if they don’t impede communication, mistakes often get a free pass. “Fewer” is retreating into formal language, along with “all right” (as opposed to “alright”). Hearkening back to the examples I gave earlier, “less” is most often used incorrectly in reference to time, money, distance, and weight. One of the biggest takeaways from the Guide is its concluding note: People probably started using “fewer” properly more often so that there would be less ambiguity in phrases like “less promising results,” where it could mean there were fewer promising results or that the results were less-promising than others.
Webster’s Dictionary of English Usage focuses on the rules of prescriptivists vs. the observations of descriptivists. The Dictionary agrees about the prescriptive rules of “less” for things that are measured (although that is a new way of putting it) and “fewer” for things that are counted. They also note that usage is different; “less” often encompasses both usages. The first note about the properness of “fewer” came up in 1770 from Robert Baker. Before Baker, “less” had been used for pretty much everything. A much later note (1988) from Trimmer & McCrimmon adds that “less” can also be used for “abstract characteristics”—like the phrase “less ambiguity,” which I used earlier. The Dictionary also agrees on the most common misuses of “less”: “distances, sums of money, units of time, and statistical enumerations.” Interestingly, the Dictionary says that “less” is preferred in these cases. (Otherwise, native speakers would be able to use their ears to tell when either is appropriate.) It also notes that misusing “less” is more common in speech than writing.
Garner’s Modern English Usage is quite interesting in that it explains why “less” is actually correct (not just “preferred”) in the few cases previously mentioned. “Less” is used instead of “fewer” in cases where count nouns function as mass nouns because “the units are so very numerous or they aren’t considered discrete items (the idea of individual units becomes meaningless).” Here the examples of time and money are given. Apparently you can’t say “fewer than 60 minutes” because the amount of time is fractionalised. If it’s thought of as a whole unit, you can use “fewer,” such as with “fewer days abroad.” Naturally, Garner also makes note of the popularity of “less” in modern times, and both Garner and Webster note the popular example of checkout signs in supermarkets saying “15 items or less.”
These sources generally agree on the proper usage of “less” and “fewer” and report on the popularity/misuse of “less” in both spoken and written contexts. They all provide a list of cases where “less” is commonly used instead of “fewer,” although only the last source says that this exception is correct (as a rule). Each source provides unique details on the issue. It can be concluded that “less” ought to be used (from the prescriptivist standpoint) with collective and mass nouns while “fewer” is reserved for count nouns, although in informal speech and written work, “less” is often used where “fewer” should be. There are exceptions (time, money, etc.) but it is still contested whether these uses are correct or not.
If you have any opinions on the matter, I’d love to hear them!