The Oxford Dictionaries gave the following rules that will help us learn how to use ‘less and fewer’.
Fewer and less both mean to decrease the amount of something:
(a) Fewer – is the comparative form of few, so if you’re talking about a smaller number of people or things, you should use fewer with a plural noun (e.g. houses, dogs, clothes, mistakes).
√ Less economic freedom means fewer jobs.
√ Fewer students are opting to study science-related subjects.
(b) Less- means ‘a smaller amount of; not as much’. If you’re referring to a smaller amount of something that can’t be counted or that doesn’t normally have a plural (nouns such as money, air, time, rain, or music), then less is the word you should choose.
√ It’s a better job but they pay you less money.
√ People want to spend less time in traffic jams.
The general rule is that if you’re talking about an amount of something that can’t be counted, it’s less and if you’re talking about a number of people or things, it’s fewer. Another way to remember the main distinction is to think along the following lines:
not as much = less
not as many = fewer
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