One reduces uncountable things, the other reduces countable things.
"Less" is used with uncountable nouns: "less water", "less time", "less effort". It refers to a quantity without discrete units you can count individually.
Try to eat less sugar.
I have less time than I thought.
She sleeps less than eight hours a day.
"Fewer" is used with plural countable nouns: "fewer people", "fewer cars", "fewer mistakes". If you can count the items one by one, use "fewer", not "less".
There are fewer students this year.
He makes fewer mistakes now.
Fewer people voted in this election.
Common mistake
Trick to remember it
Quick rule
Did you know?
US supermarkets have famous express checkout signs saying "10 items or less", grammatically it should be "10 items or FEWER" because items are countable. This controversy has been active since the 1970s and has generated debates between prescriptivist linguists (who insist on "fewer") and descriptivists (who accept "less" as a colloquial variant). Some supermarkets like Whole Foods changed their signs to "10 items or fewer" to comply with the formal rule. In everyday informal speech, "less" with countable nouns is so widespread that many modern linguists consider it an accepted variant in oral contexts, though in formal and academic writing "fewer" remains the correct choice.
Exercise 1 · Pick the right one
There are _____ students in the class this year. Two have transferred.
Exercise 2 · Pick the right one
I have been drinking _____ coffee lately. It was making me anxious.
Exercise 3 · Form the word
One is for things you can count, the other is for things you cannot count.
'A' is used before consonant sounds; 'an' is used before vowel sounds.
'A lot of' works for countable and uncountable; 'many' only for countable; 'much' only for uncountable.
'According to' cites an external source; 'based on' indicates the evidence behind a claim.
Bryan López
Founder of SpeakUP Academy