Less is for mass nouns, like grass, that don't take "a" in the singular, and have no plural. There is some grass, there is less grass. Less butter. Fewer pats of butter. Fewer blades of grass.

Fewer grassES grow in this region. Now it is a count noun. As in, barley is a grass.


Fewer is for count nouns, like dog, that can take "a" in the singular, and have a plural. There is a dog, there are some dogs, there are fewer dogs.

Both use More. More dogs, more grass. Fewer people. And so on.