All nouns are either countable (can be counted and be referred to by numbers) or uncountable (cannot be counted and are referred to by amounts).
Countable nouns are counted by adding an “s” to the end of the noun, so that 1 cats becomes, for example, 3 cats and 1 girl becomes 45 girls. However, there are nouns that are counted by other irregular forms such as 1 fungus –> 3 fungi, 1 ox –> 5 oxen, 1 axis –> 6 axes, 1 child –> 20 children, 1 phenomenon –> 2 phenomena, and so forth.
For a comprehensive list of the irregular countable nouns in English, please refer to the following link: http://english-zone.com/spelling/plurals.html
Countable nouns normally take singular verbs when they are singular and plural verb when they are plural.
Uncountable nouns are usually concepts or substances that cannot be divided or abstract ideas. For example, greatness is an idea or quality that does not have a plural form. You can count the things that make someone great, but you cannot count the greatness itself. Water is also uncountable. You cannot take hold of water to count it as it will be spilt out of your hands. The only way you can count water is by putting it in bottles, and in that way you will be counting the bottles in which the water is stored and not the water itself.
For a comprehensive list of the uncountable nouns in English, please refer to the following link: http://ieltsliz.com/uncountable-nouns-word-list/
Uncountable nouns always take singular verbs.