c) When do we use the word "less"? How about "more" ?

Answer:
According to usage rules, fewer is only to be used when discussing countable things, while less is used for singular mass nouns. For example, you can have fewer ingredients, dollars, people, or puppies, but less salt, money, honesty, or love. If you can count it, go for fewer.
The most common use of 'more' is in the comparative form. 'More' is used with adjectives of more than one syllable - with the exception of adjectives ending in 'y' - to express that there is more of a particular quality.
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Answer:
We use less, when something is not as plenty as the others.
We use more, when something is plentier than other things.