Countable nouns are things that can be counted and usually have both singular and plural forms like chairs, books, and pens. Uncountable nouns are things that cannot be counted and generally do not have plural forms like air, sand, and paint.
Countable nouns are usually preceded by an article like the, a, or an.
There's a chair in the corner. The report is on my computer.
Uncountable nouns and plural countable nouns are not preceded by the article a or an, but are often preceded by the article the. In many cases, no article is needed.
x There's a sand in my shoe. There's sand in my shoe. (no article needed)
x A doors on this floor are unlocked. The doors on this floor are unlocked.
The water in the ocean looks brown. The house has been closed up for a while, so the air is musty.
Many times people will say things like, "I'd like a coffee" or "Let's get a beer." In these cases, people are referring to an unspoken yet implied amount that generally would be countable such as a cup, a bottle, or a mug.
I'd like a [cup of] coffee. Let's get a [mug] of beer.
Put it all together
Practice using countable and uncountable nouns with this exercise sheet. You can also find it and other exercises here.