Countable nouns are things you can count such as cats, boys, books, and etc. Uncountable nouns are things that cannot be counted such as water, air and formerly things such as salt, pepper, coffee etc. Singular countable nouns must have an "a" or "an" or a delimiter such as "my" or "this." Many uncountable nouns have recently become countable such as "coffee," "beer," "coke," "sugar" and others. This is due to the container craze and packaging.

In choosing "a" or "an" consider the sound (not the spelling) of the following word.
Use the article a before all consonant sounds, including sounded h, long u, and o with the sound of w (as in one).