One of the most common grammar and usage questions I receive from students is this: How do I know whether to use "less" or "fewer"? It's an important question; using these words properly can mean the difference between sounding intelligent or seeming uneducated. No one wants to ruin a good impression with a potential employer, date, or admissions interviewer by making the wrong choice in a matter that is actually quite simple.
Here is a good test to help decide which word is more appropriate: Will the word be describing a countable noun--or will it be describing a noun that represents a group, collective, or abstract concept? If the noun is countable, then use "fewer". By way of example, it is appropriate to say, "Since I took a cut in pay, there are fewer dollars coming home each week." Another example is to say, "It is amazing that, as I grow older, it seems there are fewer hours in a day."
When speaking of a more abstract concept, group or generality, it is correct to use the word "less". Contrast the previous paragraph's sentences with this one for elucidation: "Since I took a cut in pay, there is less money coming home each week." Similarly, it is appropriate to say: "It is amazing that, as I grow older, it seems there is less time in a day."
"Dollars" is a countable noun. It requires the word "fewer" as a descriptor. "Money," however, is a more abstract or general concept and thus it requires the word "less." The difference is also evident in the sentence, "It is amazing that, as I grow older, it seems there are fewer hours in a day." Hours are specific and countable segments, but time itself is a more abstract concept and needs the word "less." (Because English words can have multiple meanings, it is important to make one distinction here regarding time. When using the word "time" to mean "occasion" or "incident," e.g., "Yesterday my mother called at least 10 times," it does become a countable noun. Hence, it is correct to say, "Today she called fewer times."
In closing, let me share one more tip. Note in the examples above that subject/ verb agreement will also differ for the nouns used with the words "fewer" and "less". Because the word "fewer" implies a true plurality, any noun it describes will require a plural verb. On the flip side, nouns described by "less" will demand singular verbs.