A countable noun is a common noun that has singular and plural forms and can be modified by a number.

The opposite of a common noun is called a mass noun, which does not have different singular and plural forms, nor can it be modified by a number.

As a general rule, words referring to objects and people are countable nouns, and words referring to liquids (water, juice), powders (sugar, sand), and substances (metal, wood) are mass nouns.

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When you travel you have two suitcases. Suitcases are the same as luggage, but you cannot say "two luggages" as luggage is a mass noun. When you travel you have luggage, or two pieces of luggage. Mass nouns use measure words like pieces of to make plurals.

You want to build a bookshelf so you buy eight boards made of wood. "Wood" is a mass noun, so it is incorrect to say you have "eight woods," but you can say you have eight pieces of wood.

Here is a list of some more mass nouns: advice, air, art, blood, butter, data, deodorant, equipment, evidence, food, furniture, garbage, graffiti, grass, homework, housework, information, knowledge, mathematics, meat, milk, money, music, notation, paper, pollution, progress, sand, soap, software, sugar, traffic, transportation, travel, trash, water

There are some words that are both countable nouns and mass nouns. You leave some papers on the desk, by which you mean you leave some specific documents. If you leave some paper on the desk, you mean you left a package of paper or just some paper in a general sense.

Further Learning
Search for some mass nouns on Yabla English and see how they are used in context.

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A noun is a word for a person, place, or thing that can be the subject of a verb. One of the first things you learn in a new language are nouns. Different languages have different ways to make a singular noun plural.

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In English, the most common way to make a noun plural is to add the letter s:
pen: pens
spoon: spoons
table: tables
letter: letters
window: windows

Nouns ending in tch, s (or ss), or x are often made plural with the letters es:
tax: taxes
match: matches
bus: buses
business: businesses

Some nouns ending in f replace the f with v, ending in ves:
shelf: shelves
knife: knives
self: selves
thief: thieves

Some nouns have irregular plurals:
man: men
woman: women
child: children
foot: feet
ox: oxen

Some nouns ending in y drop the y and are made plural with ies:
university: universities
baby: babies
But if the y has another vowel before it, then usually the plural is made by adding s:
boy: boys
monkey: monkeys

Nouns ending in o are irregular. Some end with s, some with es, and some work with both:
hero: heros or heroes
volcano: volcanos or volcanoes

Some nouns have the same singular and plural forms, and most of these are animals: moose, deer, fish, swine

Further Learning
This example from Yabla English has 5 different plural nouns, including two that are irregular:

We have brought a set of consulting tools

that include analyses, evaluation criteria,

business processes and governance recommendations.

Captions 9-11, Planview and Kalypso - Partner to Drive Innovation

 Play Caption

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Try to correctly change the four nouns to their singular form and check your work to see if you converted the two irregular nouns correctly.

For even more plurals, watch the Yabla English video English with Lauren and Matt - Parts of the Human Body.