Countable nouns vs uncountable nouns
Countable nouns are nouns(names) that we can count. For example: car, pen, book, onion.
Also Countable nouns have plural form. For example: cars, pens, books, onions, etc .
However, uncountable nouns are nouns that we can not count. For example: water, milk, air, sugar, etc.
Finally, in countable nouns we can say "one car, two cars, one pen, two pens"
But in uncountable nouns we cannot say one sugar, two sugars, one water, two waters because uncountable nouns don't take plural form.

Exercise
Write 4 countable nouns and 5 uncountable nouns.