Fewer is used to compare concrete nouns, the same words that would be counted with the word number. Less is used to compare abstract nouns, the same words that would be quantified (but not directly counted) with the word amount.
- The event drew fewer participants this year than last year. ["Participants" are individuals that can be counted.]
- Phil loves his new job even though it offers less opportunity for advancement than his former field did. ["Opportunity," in this sense, is an abstract idea.]
- As the day grew hotter, the protesters' speech became less insulting. ["Speech" is abstract.]
- While still angry, the protesters spewed fewer venomous insults as they tired in the midday heat. ["Insults" is concrete.]
- more
- "We hold in our hearts more love than hate because we believe!" shouted the pastor, spurring a frenzied chorus of "Amens!" from the enthusiastic congregation. ["Love" is abstract.]
- Janet finally concluded there were more practical reasons to stay on the West Coast than to move to New England. ["Reasons" is concrete.]
- greater
- The seating capacity of Conference Room B is greater than that of Conference Room A. ["Capacity" is abstract idea.]
- Today, it's common to have a far greater number of highly qualified applicants than available jobs--in some cases, exponentially more than ten years ago. ["Number" is concrete.]