Countable And Uncountable Nouns

In English there are countable and uncountable nouns.

As you will likely know, countable nouns can usually be singular or plural. Examples of these can be words like: "book/books", "car/cars" and "dog/dogs". Actually some countable nouns are irregular - for example, "clothes are" does not have a singular.

When a noun is countable and singular, we use a singular verb, such as "The dog is...".

When a noun is countable and plural, we use a plural verb, such as "The dogs are...".

When a noun is uncountable, we normally use a singular verb, such as "The air is...".

Sometimes nouns can be countable or uncountable, such as countably: "Waiter, there is a hair in my soup.", whilst, "Hair is cut by a hairdresser.", is uncountable.

There are various other rules and check out these reference sites for further information: 

1. http://www.learnenglish.de/grammar/articlestext,

2. https://www.englishclub.com/grammar/adjectives-determiners-the-a-an.htm.