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The word less is confused in more than one way. Please note that there are multiple groups on this page.

less used with quantities that cannot be counted
fewer used with nouns that can be counted

Examples  

Fewer students are signing up for courses in math and science.
They prefer courses requiring less study time.
less not as much in quantity (less and a singular noun)
fewer not as much in number (fewer and a plural noun)
fewest the smallest in number (fewest and a plural noun)
least the smallest in quantity (least and a singular noun)

Examples  

There is less opportunity to study Latin in public schools today than in the past.
There are fewer students signed up for Latin class this term.
This Latin class has the least chance of remaining part of the curriculum.
The fewest number of students signed up for Latin class this year.
less smaller in amount or extent; lower in rank
lesser not great in value, quantity, or size

Examples  

I put less sugar in the tea.
I was moved to a less prominent position.
Joyce Kilmer is considered one of the lesser poets.
Joyce Kilmer is a lesser-known poet.