Here's the difference, with some help from the characters in Teen Wolf.
Less is the more all-purpose word, used to modify adjectives and adverbs as well as modifying nouns.
Stiles decided to ask, since Derek looked less grumpy than usual.Compare to:
By the end of training, each time Isaac got knocked down he got up less easily.
The less information that people had about werewolves, the safer the pack was.
The fewer people knew about werewolves, the safer the pack was.The first example refers to the amount of information people have about werewolves. The second refers to the number of people who know about werewolves. When it comes to modifying nouns, this is another situation that requires determining whether you are talking about mass nouns or count nouns. We previously looked at mass and count nouns with "amount" versus "number," here.
In this case, use fewer with count nouns and less with mass nouns.
Count nouns are are things that come in discrete units that can be counted:
"Allison," Stiles whispered. "How many arrows do you have left?"Note that Lydia is measuring tablespoons of wolfsbane powder, not "three wolfsbane powder." Liquids, powders, gases, substances like wood or metal, and concepts like time and money are mass nouns that can't be directly modified by a number. So it's:
"Three," she whispered back.
"That's two fewer than we need," he said, watching the five omegas spreading out around them. Of course, Allison's deadly aim would soon turn that into three fewer wolves.
Lydia measured three tablespoons of wolfsbane powder into each explosive device. Dr. Deaton suggested, "Try two. Fewer tablespoons means more explosives."
Lydia put less wolfsbane powder into each device since they needed as many as they could get.This should be fairly easy to remember, since the word few often stands in for a number.
"How many handfuls of mountain ash are left?"
"There are only a few handfuls of mountain ash left." (”There are only four or five handfuls of mountain ash left.")
"After you finish warding that room, there will be one fewer.”
In summary, use less in most situations. But when you have a noun that can be modified by a number (or "a few"), use fewer instead.