Some English nouns are countable, that is they have a plural form; some nouns are non-countable and have no plural form; and some can be either countable or non-countable depending on what is meant. In most cases countable nouns form their plurals by adding an s. You have to consider countability when choosing articles (a, an, the) and choosing between qualifiers such as fewer and less. Here are some guidelines regarding countability.

Countable nouns

• (usually objects with a definite shape): rocks, trees, pencils, tables, doors, etc.

• (substances without definite shape): smoke, water, sand, etc.. In this case the container is what becomes plural-- for example, buckets of sand, columns of smoke
• (abstractions or qualities): honesty, knowledge, patience, happiness, etc.
• (category words): clothing, luggage, equipment, vocabulary, homework, grammar, etc.

• countable: chickens (the birds themselves), pains (the ones in your joints), metals (such as silver or gold), baseballs (what you throw), works (as in works of art)
• non-countable: chicken (the meat itself), pain (what you feel), metal (the substance itself), baseball (the game), work (the activity, what you do for a living)

• for use with countable nouns: many, few, fewer, several, a number of
• for use with non-countable nouns: much, less, little, a great deal of, an amount of
• for use with either kind of noun: some, lots of, a lot of, no, not any