Most of us just know when to use fewer, and when to use less. We hear them often enough to make it instinctive. But, as in all things, knowin why you do something makes the difference between knowing and knowing.

I'm going to make this as simple as possible:

Count nouns = things you can count = fewer
ex: Reese's pieces, napkins, books

If I ate fewer melted Reeses pieces, I would use fewer napkins, and ruin fewer books.

Easy, right? But the next one is a little tricker.

Mass nouns = things you cannot count individually = less
ex: clutter, traffic, sugar

I don't have a quippy sentence for this one, because it's just a little more abstract.
ex: Less clutter makes a more efficient kitchen.

The clutter in my kitchen consists of cookbooks and pots and pans and baking ingredients--all individual things that make a whole, but remain individual. You can't say Fewer clutter makes a more efficient kitchen.

But what about sugar, you ask? That's an even trickier one, and why I threw it in there. One can have less sugar, and fewer sugars--why? Because sugar, all those individual and seemingly uncountable grains is actually not infinite. It is a collective of one kind, and thus countable. BUT! What if you have different sugars in one bowl? Brown sugar, beet sugar, cane sugar, organic sugar, bleached sugar, confectioners sugar!!! You get the drift. So:

Less sugar makes this dish more savory.
--and
Fewer sugars make this dish more savory.
--both work.

And there's a little cheat for you, if you saw it--mass nouns aren't typically plural. Count nouns typically are. Fewer sugars, less sugar. See?

There are exceptions--as always: Time, distance and money. These take less, even though they will often be plural.
ex: That coat costs less than two hundred dollars.
ex: The distance to the animal shelter is five miles less than the distance to town.
ex: Darlene danced for less than fifteen minutes.
There's the s! Why not fewer? Because these are exceptions. Why? Because time, distance and money are all uncountable as bulk quantities, so less is correct, even though there are countable units of measure for them. (And that confuses the crap out of me every time! So just remember that time, distance and money use less and be happy.)

It's kind of the same thing for words like further and farther. (Further for metaphorical distance, farther for physical distance. One gets further from the truth, farther from home.) One is going to be the more abstract, while the other is more physical. Knowing which is which goes a long way to understanding why, and knowing why is always better than guessing.

And before I go, I wanted to share something I just found on Grammar Girl--my favorite go-to site for all things grammarly. You know that sign over the express line at the grocery store?
Ten items or less.

What do you see there? It's WRONG! It should be ten items or fewer. Why less? It's a common usage thing now, the whole prescriptive vs. descriptive thing. Fun, huh? But another story for another day. And because you've been so good and gotten this far--have a funny comic.

(click to make it bigger)