Countable nouns are refer to items or concepts that may be counted; thus, they may be either singular or plural. Although you can often tell whether a noun is countable or uncountable by its meaning, this determination is not always predictable. If you are unsure whether a given noun is countable or uncountable, check a specialized dictionary. The words laboratory, electron, and hypothesis are countable nouns (their plurals are laboratories, electrons, and hypotheses).
Use the ending -s to form the plural of most countable nouns.
Always use an article or some other determiner before a singular countable noun.
Largest jet plane holds enough fuel to drive ordinary car four times around
world.
The largest jet plane holds enough fuel to drive
an ordinary car four times around the
world.
--"Take It or Leave It," Valley Comic News
Do not use an article before a plural countable noun unless you are restricting the interpretation of the noun in some way.
For the most part, chemistry treats the atoms as if they
were the tiny yet solid balls of matter which stick together
in the various arrangements to form substances of which the
everyday world is composed.
For the most part, chemistry treats atoms as if they were tiny yet solid
balls of matter which stick together in various arrangements to form
the substances of which the everyday world is composed.
[The last phrase, "of which . . . composed," restricts the
interpretation of the word "substances."]
--Philip Ball, Designing the Molecular World
