Some nouns are known to only exist in singular forms even when they are use to refer to more than one thing. These nouns are known as 'singulare tantum' which means only singular forms. Nouns that are use to refer to a collection of things are usually in singular forms. - furniture, hair, crew.
Some nouns ending with -s are considered singular - news, statistics, logistics, physics, billiards.
Some nouns are special, there is no reason why they exist only in dingular forms. Examples - information, knowledge, milk, home work. The list of nouns that exist in singular forms only is endless.
Here are some more examples: measles, darts, crossroads, mumps, series, economics.
Nouns that are always used in the singular can be put into groups. Collective names of nouns like water, salt, furniture, and money are typically not ever put into the plural. They aren't countable so they are used with words like 'some' or 'more.' A special case of this is the word 'hair.'
When talking about a head of hair, the word hair is singular yet refers to all of the hair on the head. When someone uses the word hairs, they are using the plural of hair while talking about individual strands. The nouns information, knowledge, and homework are all other examples of nouns that are always used in the singular.