The words "less" and "fewer" both mean the same thing—they are both the opposite of more—but students of English, and even native speakers, often have trouble knowing which word to use in different situations. Sometimes you can go by ear and pick because one will sound better than the other, but there's a simple rule to make sure you choose the right word every time.
"Less" and "fewer" are both adjectives that modify nouns, but they have different uses. The basic rule to remember:
To see the rule in action, consider the following examples of correct usage of the word "less":
In each sentence above, the word "less" describes a singular noun. It's important to note that in the case of "less" and "fewer," singularity is a grammatical idea, but not necessarily a physical one. For example, you could argue that "sugar" and "money" are actually plural in real life, because there are many crystals of sugar or pennies in the pile of money. While that may be true, both "sugar" and "money" are singular when used in a sentence. For example:
As you can see, both "sugar" and "money" take the singular verb "is" and are treated as singular nouns. This is why they should be described with "less" instead of "fewer." Likewise, the "one problem" in the examples above is also singular, which is why the word "less" is correct in that case as well.
Now consider these correct examples of the word "fewer":
In each sentence above, the word "fewer" describes a plural noun. "People," "gifts" and "dollars" are all plural nouns, so they are properly described by the word "fewer." Note that in the third example, "fewer" modifies the plural noun "dollars," but "less" modifies the singular noun "money" — even though they denote roughly the same thing. This is a quirk in the English language in the way we use money as a singular noun, but it nevertheless does not affect the rule about singularity and plurality for less or fewer usage.
The most common error of this type is one you see every time you visit the grocery store. Consider the rules for the express lane:
Based on the singular vs. plural rules it's clear that this is incorrect, because "twelve items" is clearly plural. In this case, it would be correct to say "twelve items or fewer" or "fewer than twelve items."
Another common error comes when people misunderstand the singular vs. plural rule. As noted above, singular refers to the way the word or words are used in the sentence — that is, for the purposes of noun-verb agreement — rather than whether the items in question are actually plural in real life. For example:
Though on the surface "five days" seems plural because five is many more than one, you must instead think about whether or not the phrase "five days" is singular or plural within the sentence.
To apply this test, consider whether the phrase would take the verb "is" or "are":
Because "five days" works as a singular noun grammatically (it is a span of time here, not a number of days), it should be described using "less" instead of "fewer." When in doubt, you can make the right choice by applying the "is vs. are" test to decide whether your word or phrase is grammatically singular. If it is, use the word "less." If not, use “fewer."
Now you know the simple rule for working out whether to use less or fewer, you will make fewer mistakes. However, this is one of those grammatical rules that has been consistently broken over the years, so if you use the wrong one in informal writing or speech, maybe because it sounds better to your ear, that is not a serious error. Just remember to be more careful in a formal setting.