Nouns in English can be either countable or uncountable.
Countable nouns are those nouns that can be counted.
Uncountable nouns are those nouns which cannot be counted because they cannot be defined individually, but rather are part of a whole. As such, they are treated as singular (you can not make them plural by adding “-s”).
However, once we delimit these nouns by placing a in front of the noun, they become countable.
1. Countable nouns have a plural form (regular or irregular):
Uncountable nouns do not have a plural form:
2. You can use “a” or “an” with countable nouns in singular:
We cannot use “a” or “an” with uncountable nouns:
3. Numbers can be used in front of countable nouns:
Numbers cannot be used in front of uncountable nouns: