Each of the articles – a, an, the – can be used before a countable noun. Unlike a and an, the article the can come before an uncountable noun. The three articles are commonly used to begin a sentence. There are some words that can be used before the articles.
Articles before a countable noun.
The three articles are used here before a countable noun or before an adjective followed by a noun. The nouns used here are goal, priest, apple, friend, fireworks, and building.
Article before an uncountable noun.
The is the only article that is used before an uncountable noun. It can be anywhere in a sentence except at the end. The uncountable nouns used here are butter, flesh, sand, snow, cream, and skin.
We cannot use a before an uncountable noun, but we can if we have a measured quantity of the uncountable noun. For example, it's wrong to say or write a cheese. Instead, we can use a chunk/hunk/lump/piece/slice of cheese.
(For more, see List 2 - Uncountable Nouns made Countable)
Words that come before the articles.