1 Countable nouns are the names of separate objects, people, ideas etc which we can count.
We can use numbers and a/an with countable nouns: they have plurals.
a cat - three cats
a newspaper - two newspapers
Uncountable nouns are the names of materials, liquids, and other things which we do not see as separate objects. We cannot use a/an or numbers with uncountable nouns; they have no plurals.
Water (NOT a water; two waters)
Wool (NOT a wool; two wools)
Weather (NOT a weather; two weathers)
2 We cannot usually put a/an with an uncountable noun even when there is an adjective.
My father enjoys very good health. (NOT . . . a very good health.)
We’re having terrible weather. (NOT . . . a terrible-weather-)
He speaks good English. (NOT ... a good English. )
3 Usually it is easy to see if a noun is countable or uncountable.
Obviously house is a countable noun, and air is not. But sometimes things are not so clear. For instance, travel and journey have very similar meanings, but travel is uncountable (it means ‘travelling in general’) and journey is countable (it means ‘one movement from one place to another’). Also, different languages see the world in different ways. For example hair is uncountable in English, but plural countable in many languages: grapes are plural countable in English, but uncountable in some languages.
Here are some more nouns which are uncountable in English, but countable in some other languages, together with related singular countable expressions.
a place to live or stay (NOT an accommodation)
a piece of advice (NOT an advice)
a loaf: a roll
a piece of furniture
a blade of grass: a lawn
a piece of information
a flash of lightning
a piece of luggage; a case: a trunk
a note: a coin; a sum
a piece of news
a step forward
a piece of research: an experiment
a piece of rubbish
a piece of spaghetti
a clap of thunder
an aching tooth
a journey: a trip
a job: a piece of work
Note: A headache is countable.
4 Many nouns have both countable and uncountable uses.
I’d like some white paper. (uncountable)
I’m going out to buy a paper. (= a newspaper— countable)
The window’s made of unbreakable glass. (uncountable)
Would you like a glass of water? (countable)
Could I have some coffee? (uncountable)
Could we have two coffees, please? (= cups of coffee— countable)
She’s got red (uncountable)
I’ve got two white hairs. (countable)