(a) Canada is a country.
Singular means “one.”
In (a): Canada = a singular noun
is = a singular verb
country = a singular noun
(b) Mexico is a country.
A frequently comes in front of singular nouns.
In (b): a comes in front of the singular noun country.
A is called an “article.”
(c) A cat is an animal.
A and an have the same meaning. They are both articles. A is used in front of words that begin with consonants: b, c, d, f, g, etc.
Examples: a bed, a cat, a dog, a friend, a girl
An is used in front of words that begin with a, e, i and o.*
Examples: an animal, an ear, an island, an office