It is difficult to know if a noun is countable or uncountable. The easiest way to know if a word is countable or uncountable is check a dictionary. But there are some ways to guess and some rules to help you.

Countable Things

Countable nouns are always about the same shape or size. It is easy to think of "one" or "a". They have a limit or edge to their shape.

Another way to know if something is countable is if you and someone else think about a countable thing, they will be about the same:

Lastly, if you can draw a picture of "one" then the thing is countable: 

Uncountable Things

Uncountable things can be many different shapes and sizes, and thinking of "one" or "a" is hard:

"Water" is uncountable because there is no clear shape or limit of one. Is "one water" a drop of water, a glass of water, a pool, a wave, a river, a lake, an ocean, rain, a sea...?

The key is that my idea of "one water" may easily be different to your idea of "one water". When we think about water, this might happen:

There are ways to count uncountable things.

Learn more about counting uncountable nouns

Groups and Members

Often the name of a group of things is uncountable and the members of that group are countable: