Answer: Fewer & Less
rykaine asks: When is it appropriate to use "fewer" vs. "less"?
With examples from Fullmetal Alchemist, Stargate: SG-1, and Harry Potter
The Fewer Rules You Have, The Less Confusing It Will Be
The rules defining fewer and less are quite simple and easy to commit to memory.
Fewer
Use fewer to describe countable things.
After Daniel hired Nyan as a research assistant there were fewer piles of clutter in his office.
Thanks to Hawkeye's influence, Roy had fewer female underlings.
Fewer still were Harry’s chances of finding the Lost Diadem.
Less
Use less to describe uncountable quantities (a group of things that cannot be counted individually), collective amounts (a group that defines many things), and degree.
uncountable quantities After Daniel hired Nyan as a research assistant there was less clutter in his office.
collective amounts Thanks to Hawkeye's influence, Roy’s pool of female underlings was significantly less.
degree Harry had less hope than ever of finding the Lost Diadem.
Countable and Uncountable Quantities
Another way to think of the rules is how the Grammar Girl explains them. Namely that fewer is used with "count nouns" and less is used with "mass nouns."
Count nouns are things that can be counted individually such as "piles," "underlings," or "chances."
Mass nouns are things that you can't count individually such as "clutter," or "tape."
In the examples above, you can count the fewer piles of clutter, but not the clutter.
After Daniel hired Nyan as a research assistant there were fewer piles of clutter in his office.
After Daniel hired Nyan as a research assistant there was less clutter in his office.
The Grammar Girl's litmus test for "mass nouns" is that you can't put an "s" on the end to make the noun plural. For example, you wouldn't say that Daniel's desk has clutters.
While thinking of count nouns and mass nouns can help identify how to use fewer and less, there are other rules to remember. The last two examples have less modifying count nouns ("pool" and "hope"), so why do we use less in these cases? The difference in these examples has to do with intent.
When Intent Means Everything: Collective Amounts & Degree
Less is also used to describe collective amounts and degree and can be used with "count nouns" in these cases.
The technological improvements of the Aschen mean that there is less illness in the world.
While a number of illnesses can be counted, in this case "illness" is being used to define a collective amount, that is to say in this sentence, the amount of disease has decreased. If illness were being used as something to be counted, the sentence would be: The technological improvements of the Aschen mean that there are fewer illnesses in the world. This sentence means that the number of diseases has decreased.
Remus had less than two years with Sirius before he was taken away again.
In this example "years" is plural even though "less" is being used! That is because money, time, and distance are typically described with "less" because they describe collective amounts. A number of coins, hours, or feet can be discussed, using fewer such as, As exams grew closer, Hermione felt there were fewer hours in the day. However, in talking about an amount of money, time, or distance, use "less" to define the collective unit. (This is also why in mathematics class you say that 4 is less than 5.)
Ed has less control over his temper than Mustang.
"Less" here is describing the degree of control Ed has. The earlier example, Harry had less hope than ever of finding the Lost Diadem, describes the degree of hope Harry has.
Ways To Test
You can test if you should use fewer or less by substituting the modifiers. It's often much easier to determine if you should use "many" or "much" in a sentence.
Fewer/Many
After the battle, Ba'al's forces were fewer.
After the battle, Ba'al's forces were many.
Less/Much
After Daniel hired Nyan as a research assistant there was less clutter in his office.
After Daniel hired Nyan as a research assistant there was much clutter in his office.
This doesn't work with every example, but can fit most examples with a bit of word rearrangement.
Fewer/Many
Havoc tended to have fewer dates than Mustang.
Havoc tended to have many more dates than Mustang.
Less/Much
After Bagman refused to make good on their bet, Fred and George had less than a knut to their names.
After Bagman refused to make made good on their bet, Fred and George had much more than a knut to their names.
Fewer Aggravations, Less Misuse
The typical aggravation regarding less and fewer can be found in the express check-out at grocery stores. Signs declare that you can check out if you have "10 items or less," however as we've learned, since your items are countable, "fewer" should be used on these signs.
So, as a parting reminder: in grocery stores you can go through the express check-out if you have 10 items or fewer, but the till may not be able to make change for purchases less than a dollar.
Further Reading on Less and Fewer:
Wisdom from the Grammar Goddess: The Appropriate Use of Less and Fewer
English Plus: Fewer or Less?
Grammar Girl: Less Versus Fewer
With examples from Fullmetal Alchemist, Stargate: SG-1, and Harry Potter
The Fewer Rules You Have, The Less Confusing It Will Be
The rules defining fewer and less are quite simple and easy to commit to memory.
Fewer
Use fewer to describe countable things.
After Daniel hired Nyan as a research assistant there were fewer piles of clutter in his office.
Thanks to Hawkeye's influence, Roy had fewer female underlings.
Fewer still were Harry’s chances of finding the Lost Diadem.
Less
Use less to describe uncountable quantities (a group of things that cannot be counted individually), collective amounts (a group that defines many things), and degree.
uncountable quantities After Daniel hired Nyan as a research assistant there was less clutter in his office.
collective amounts Thanks to Hawkeye's influence, Roy’s pool of female underlings was significantly less.
degree Harry had less hope than ever of finding the Lost Diadem.
Countable and Uncountable Quantities
Another way to think of the rules is how the Grammar Girl explains them. Namely that fewer is used with "count nouns" and less is used with "mass nouns."
Count nouns are things that can be counted individually such as "piles," "underlings," or "chances."
Mass nouns are things that you can't count individually such as "clutter," or "tape."
In the examples above, you can count the fewer piles of clutter, but not the clutter.
After Daniel hired Nyan as a research assistant there were fewer piles of clutter in his office.
After Daniel hired Nyan as a research assistant there was less clutter in his office.
The Grammar Girl's litmus test for "mass nouns" is that you can't put an "s" on the end to make the noun plural. For example, you wouldn't say that Daniel's desk has clutters.
While thinking of count nouns and mass nouns can help identify how to use fewer and less, there are other rules to remember. The last two examples have less modifying count nouns ("pool" and "hope"), so why do we use less in these cases? The difference in these examples has to do with intent.
When Intent Means Everything: Collective Amounts & Degree
Less is also used to describe collective amounts and degree and can be used with "count nouns" in these cases.
The technological improvements of the Aschen mean that there is less illness in the world.
While a number of illnesses can be counted, in this case "illness" is being used to define a collective amount, that is to say in this sentence, the amount of disease has decreased. If illness were being used as something to be counted, the sentence would be: The technological improvements of the Aschen mean that there are fewer illnesses in the world. This sentence means that the number of diseases has decreased.
Remus had less than two years with Sirius before he was taken away again.
In this example "years" is plural even though "less" is being used! That is because money, time, and distance are typically described with "less" because they describe collective amounts. A number of coins, hours, or feet can be discussed, using fewer such as, As exams grew closer, Hermione felt there were fewer hours in the day. However, in talking about an amount of money, time, or distance, use "less" to define the collective unit. (This is also why in mathematics class you say that 4 is less than 5.)
Ed has less control over his temper than Mustang.
"Less" here is describing the degree of control Ed has. The earlier example, Harry had less hope than ever of finding the Lost Diadem, describes the degree of hope Harry has.
Ways To Test
You can test if you should use fewer or less by substituting the modifiers. It's often much easier to determine if you should use "many" or "much" in a sentence.
Fewer/Many
After the battle, Ba'al's forces were fewer.
After the battle, Ba'al's forces were many.
Less/Much
After Daniel hired Nyan as a research assistant there was less clutter in his office.
After Daniel hired Nyan as a research assistant there was much clutter in his office.
This doesn't work with every example, but can fit most examples with a bit of word rearrangement.
Fewer/Many
Havoc tended to have fewer dates than Mustang.
Havoc tended to have many more dates than Mustang.
Less/Much
After Bagman refused to make good on their bet, Fred and George had less than a knut to their names.
After Bagman refused to make made good on their bet, Fred and George had much more than a knut to their names.
Fewer Aggravations, Less Misuse
The typical aggravation regarding less and fewer can be found in the express check-out at grocery stores. Signs declare that you can check out if you have "10 items or less," however as we've learned, since your items are countable, "fewer" should be used on these signs.
So, as a parting reminder: in grocery stores you can go through the express check-out if you have 10 items or fewer, but the till may not be able to make change for purchases less than a dollar.
Further Reading on Less and Fewer:
Wisdom from the Grammar Goddess: The Appropriate Use of Less and Fewer
English Plus: Fewer or Less?
Grammar Girl: Less Versus Fewer