Articles are words used before nouns to make it clear exactly what the noun refers to. English has two types of articles: definite and indefinite.
The Definite Article
The definite article is the word the. The definite article limits the meaning of a noun to one particular thing. For example, your friend might ask, “Are you going to the party this weekend?” The definite article tells you that your friend is referring to a specific party that both of you know about. The definite article can be used with singular, plural, or uncountable nouns.
The Indefinite Article
The indefinite article takes two forms. It’s the word a when it precedes a noun that begins with a consonant. It’s an when it precedes a noun that begins with a vowel. The indefinite article indicates that a noun doesn’t refer to a particular thing. For example, you might ask your friend, “Should I bring a gift to the party?” Your friend will understand that you are not asking about a specific type of gift or a specific item. “I am going to bring an apple pie,” your friend tells you. Again, the indefinite article indicates that she is not talking about a specific apple pie. Your friend probably doesn’t even have any pie yet. The indefinite article only appears with singular nouns.
Choosing a or an
There are a few exceptions to the general rule of using a before words that start with consonants and an before words that begin with vowels. The first letter of the word honor, for example, is a consonant, but it’s unpronounced. The first sound of the word is a vowel sound; therefore, use an.
Similarly, when the first letter of a word is a vowel but is pronounced with a consonant sound, use a.
This holds true with acronyms and initialisms, too: an LCD display, a UK-based company, an HR department, a URL.
Article Before an Adjective
Sometimes an article modifies a noun that is also modified by an adjective. The usual word order is article + adjective + noun. If the article is indefinite, choose a or an based on the word that immediately follows it.
Zero Article and Idiomatic Use
Occasionally, idiomatic usage allows you to omit articles altogether before certain nouns. The article is implied but not actually present. This implied article is sometimes called a “zero article.” Often, the article is omitted before nouns that refer to abstract ideas.
All or All of and The
When using the words all or all of and the together, all must come first.
Articles and Pronouns
Do not use an article with a pronoun.