I
pronoun
BAD : If there were less cars on the roads, there would be less accidents.
GOOD : If there were fewer cars on the roads, there would be fewer accidents.
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Use less with an uncountable noun and fewer with the plural form of a countable noun: 'Less traffic would mean fewer accidents.'
Note that less is widely used in place of fewer in everyday conversation, but careful users regard this usage as non-standard.
II
adverb
BAD : Teachers today are less stricter than they used to be.
GOOD : Teachers today are less strict than they used to be.
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The comparative ending ( -er ) is NEVER used with less or more .