This is probably because most learners will be aware of the difference between countable nouns (such as apple, dog, and child) and uncountable or mass nouns (such as rice, milk, and time), and this is useful for understanding the basic rule:
… use less for things you can’t count (uncountable/mass nouns):
I use less sugar than the recipe recommends.
Modern cars use less fuel.
… use fewer for things you can count (countable nouns).
Fewer people use libraries nowadays.
This process leads to fewer errors.
Most first language speakers simply don’t think of nouns in that way. The result is that many of them don’t know that there’s any difference between ‘less’ and ‘fewer’. Others know, but don’t really care. However, there is a third group that does know, does care, and gets very angry indeed when they are used incorrectly. Read the rest of this entry ?