For native English speakers the classification of nouns as either 'countable' or 'uncountable' is largely of no importance or significance because they generally know from experience the common and proper usage rules for different types of nouns.
But the challenge of 'plurals' for some English words, especially for 'uncountable' nouns, is not an easy one for speakers whose first language is not English, nor for those who are still learning English.
1. Nouns can be countable or uncountable.
1.1 Countable nouns relate to things which can actually be counted. These nouns can then have a/an or the before them and can have both a singular and a plural form:
e.g.: There is a lion, three elephants and a flock of parrots in the forest.
1.2. Uncountable nouns are words for things which cannot literally be counted. Therefore they cannot have a/an (which means 'one') before them. They have no 'plural' and are not, and cannot, be used in a plural sense.
e.g.: Unfortunately my money is in my luggage which I've left at our holiday accommodation.
2. Some, any and much.
1.1 Some, any and much can be used with uncountable nouns. Have you any luggage? I haven't got any equipment. How much money have you brought?
Some more uncountable nouns: advice, air, baggage, behaviour, bread, chewing gum, furniture, garbage, influenza, information, knowledge, lightning, measles, milk, news, publicity, research, rice, rubbish, sand, sugar, traffic, water, weather, work.
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MIXED USES
Some words can be either countable or uncountable depending on how the word is being used in context.
e.g. paper, in a general sense, is uncountable. You can buy some paper, you can even buy 500 sheets of paper, but you can't buy a paper! Well, you could! But that would mean a newspaper! -and 'newspaper' is countable.
DRINK, DRINKING AND RELATED EXPRESSIONS
Some nouns which are clearly uncountable in a general sense are expressed in a countable way when used in a drinking context.
For example, if you need some groceries from a store and want to buy some coffee (uncountable), you do not ask for 'a coffee'. You would tell them how many tins, jars, packets, grams, kilos or pounds of coffee you want.
However, in a restaurant your friend might order 'two coffees', meaning two cups of coffee.
And if they ask you if you take sugar, you might answer, 'Yes, two sugars for me, please', meaning two spoonfuls of sugar.
SOME UNCOUNTABLE NOUNS ARE PLURAL.
These have no singular form. e.g. clothes, groceries, thanks, jeans, police, trousers, scissors.
e.g. I sent a letter of thanks to the police.
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It is not possible within the scope of WikiAnswers to provide a complete and comprehensive guide to 'countable' and 'uncountable' nouns, but for more information, see Related links below this box.