Fewer is the comparative of few (used with countable, plural nouns).
Less is the comparative of little (mostly used before uncountable, singular nouns).
- A secretary makes less money than a manager.
- Last year we went to the seaside at every weekend, but now we have less time.
- There were fewer eggs in the fridge than we had hoped.
- I have fewer books than my grandfather.
Less may also be used before plural nouns in an informal style, although this is not typical:
- These days I’ve got less/fewer problems than I used to have.
Less and fewer are used with the preposition of before determiners (the, my, this, etc.) and pronouns.
- I wish my wife spent less of her money on expensive clothes.
- There are fewer of us at the college reunions each year.
- Fewer of the applicants were wearing a tie to the interview than we expected.
- Did you drink half the wine? – No, I drank less of it.
Of is not used before nouns without determiners.
- If you want to lose weight, you should eat less chocolate and bread.
- Fewer people eat only traditional food these days.
- Peter has fewer friends than his brother.
- I don’t think less time than a week would be enough for this job.
Less and fewer can stand on their own, that is without a noun, if the meaning is clear or as indefinite pronouns.
- People go to church nowadays, too, but fewer/less than 50 years ago.
- If you work less, you will earn less.
- Have you brought a basket of apples? – No, we brought fewer/less.
- Unfortunately, I often sleep less than enough.
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