Countable Noun

Countable nouns are things which we can separate:
This pen is not the same as that pen.
Quick, give me a phone! Any phone is okay!

If a countable noun is singular (i.e. just one), we must put a [determiner] before it:
my phone, her phone, this phone, that phone, each phone, every phone
Countable, singular nouns are the only type of nouns before which we can put a or an.

If a countable noun is plural (i.e. more than one) or if a noun is uncountable, we do not have to put a [determiner] before it. This is one way to recognise a countable noun.