Теорія:

Study the information about countable and uncountable nouns. Part 1

Anything that can be counted, whether singular - a dog, a house, a friend, etc. or plural - a few books, lots of oranges, etc. is a countable noun.
 
Anything that cannot be counted is an uncountable noun. Uncountable nouns are not individual objects, they are always singular.
 
1. Countable nouns can be singular and plural.
 
A car - cars
 
New cars are expensive.  
 
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2. With singular countable nouns we use a/an.
 
I have not got a key.

3. Uncountable have only one form.
 
Music, money, salt, water
  
I like rock music.
  
Use the verb in singular.
  
4. Don’t use articles a/an with them.

But you can say :

A glass of water
A piece of cheese
A bowl of rice
A can of oil
A  bottle of milk
A cup of coffee
A bar of chocolate
 
I will buy a bar of chocolate.  
 
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