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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: