One of the most common errors I correct is the misuse of the words less and fewer. When should you use “less” and when should you use “fewer”?
Use fewer when referring to a number. If you can count the items there are fewer, not less, of them. Example: Bob took off fewer days this year than he did last year.
Use less when referring to something you cannot count. Example: There was less traffic today than there was yesterday.
Count (or quantifiable) nouns versus mass (or singular) nouns
Less and fewer are synonyms. They are both comparative words, and they both mean the opposite of more.
Rule: Use fewer with count nouns and less with mass nouns.
It is easy to remember! Count nouns are simply things that you can count. A mass noun is something you cannot count independently.
Another way to remember: Mass nouns cannot be pluralized. In the above example, we can say “days” but we can’t say “traffics.” Fewer days, less traffic.
The exception: money. Bob earned less than $5,000 last year.