a car / some money (countable/uncountable 1)
You can use one/two/three (etc.) + countable nouns (you can count them):
Countable nouns can be singular (= one) or plural (= two or more):
You can use the singular (car/house/key etc.) alone. You need a/an (→ Unit 64):
You cannot say one/two/three (etc.) + these things: one water two musics
Uncountable nouns have only one form:
You cannot use a/an + uncountable nouns: a money an advice
But you can say a piece of ... / a glass of ... etc. + uncountable noun: