The usage of the comparatives “fewer” and “less” can be better understood in terms of whether the nouns they are modifying are countable or noncountable. In English, as we all know, something is countable if we can figure out without great difficulty how many of it there are; we then use “number” as an indefinite measure for it, as in “the number of candidates,” “a number of job openings,” “the number of parties involved,” and “the growing number of complaints.” In contrast, something is noncountable if it is in bulk form and counting its constituent units would be insufferably difficult or impossible; we then use “amount” as a measure for it, as in “the amount of potable water,” “a great amount of energy,” “a great amount of patience,” and “a large amount of dissatisfaction.”
Now, the word “fewer” is used as a comparative for plural count nouns, or things that use “number” as measure; thus, we say, “There are fewer candidates for club president this year,” “We find fewer job openings in the classified ads these days,” “Fewer parties took interest in the public bidding for the irrigation project,” and “The city police reported fewer robberies in 2009 than those of the previous year.” It may not be immediately apparent to nonnative English speakers, but native English speakers would find it grammatically odd if someone used “less” instead of “fewer” in those four sentences.
On the other hand, “less” is used as a comparative for singular mass nouns, or things that use “amount” as a measure; thus, we say, “We consumed less water this month than last month,” “Our factories should consume less energy to remain competitive,” “The manager proved to have less patience with the student interns than we anticipated,” and “The latest consumer survey shows less dissatisfaction with our products than last quarter’s.” If one used “fewer” in place of “less” in those four sentences, native English speakers would notice something grammatically wrong with the statement.
There are some notable exceptions to these prescriptions, though. When plural count nouns are thought of as an aggregate, “amount” instead of “number” can be used as a measure for them, as in these examples: “We will supply you with whatever amount of Hawaiian pineapples you will require.” “No amount of words will convince a rational-thinking person that Earth is only 5,000 years old.” Also, in certain cases, it is grammatically correct to use a singular mass noun in the plural-count sense, like “food” in the following sentence: “We need to reduce the number of kilos of food we buy weekly.”
