Strictly speaking, this isn’t a grammar lesson; it’s a vocabulary lesson. But I’m including it anyway because it was requested.
Less and fewer are commonly confused, and there are plenty of instances where the error has been institutionalized to the point that it’s believed to be correct (For example: “15 items or less” at the grocery store). Keeping these two adjectives straight is really pretty easy, if you just remember two simple rules:
1. Less is for mass or abstract objects (time, money, food), while fewer refers to specific, countable objects (hours, dollars, apples).
2. Less is used with singular words or plurals that don’t end with “s,” while fewer is used with plurals that end with “s.”
What does this mean? Here are some examples:
I plan to watch less TV by saving fewer shows to my DVR.
There are fewer products being manufactured in the Rust Belt, so there is less employment available.
I want to eat less, so I need to bake fewer cakes.
Got it? The “s” is really the easiest way to remember: Words that end in “s” are described with “fewer.” Of course, like any grammatical (or vocabulary) rule, there are always exceptions. And there are also cases where common usage trumps “correct” usage—”10 items or less” might be incorrect, but don’t be surprised if you’re told to get a life when you inform your local grocery store.