This one is so easy, and yet so rarely followed. Use “fewer” for things that can be counted. Use “less” for things that cannot be counted.
- Oranges can be counted. Therefore, I bought fewer oranges this week than I did last week. Fruit is the collective quantity, so this week, I have less fruit (but fewer individual fruits).
- People can be counted. Company A has fewer employees than Company B. School A has fewer students than School B. Some reality show has fewer contestants than last year.
- Emotions cannot be counted. I have less enthusiasm than she does regarding word choice. I have less anger than she does about grammar.
- Liquids are tricky. There are fewer glasses of water, but there is less water overall. (Because you can count glasses.) Water is the collective quantity.
- Time is tricky. I worked fewer hours this year, resulting in being paid for less time. Again, time is the collective quantity and hours are the countable units.
Hey, grocery store? “Twelve items or fewer.” Whole Foods may be expensive and pretentious, but they know their grammar. TBS? “More movies, fewer commercials.” Hearing “More movies, less commercials” in EVERY SINGLE PROMO is infuriating. How many people signed off on that so that such an obvious, glaring mistake gets aired during every damn commercial break?