There are two types of articles in English. They are the
definite and the indefinite articles.

Definite Article

Indefinite Article

A/AN

In general, the definite article is used before specific nouns.
The indefinite article is used before non-specific nouns.

Example:

I need a pen. (Any pen)
I need the pen on the table. (A specific pen)

The Definite Article (The)

The definite article is used as follows:

Specific Nouns

The teacher is Canadian. (A Specific teacher)

Nouns known by the speaker and listener

The teacher is Canadian.

(both the speaker and the listener know what teacher.
Perhaps the teacher of their class.)

Plural Countable Nouns

The books are on the table.
The boys like hockey.

Uncountable Nouns

The water is on the table.
The sugar is sweet.

General Ideas

The computer has improved the speed of business.
The cell phone makes business faster.

Adjectives as Nouns

New taxes will benefit the rich.
Cry for the departed.

Proper Nouns for Geographic Locations
Canals, deserts, forests, oceans, rivers, seas, plural
islands, lakes, countries

The Amazon River
The Indian Ocean

Public Places

The bank
The post office

The Indefinite Article (A/AN)

The indefinite article is "a" or "an" and is used differently than
the definite article. It is used in the following situations:

Single Count Nouns

A dog is barking.
There is a pencil on the table.

Unknown Nouns

A man is on the phone for you. I don’t know who it is.

General Statements

A doctor must study for many years.

A vs. An

A and An are indefinite articles used in the same situations.
However, the word that follows affects which article to use. If
the word starts with a consonant sound, use "a".

A dog
A man
A chair

If the next word starts with a vowel sound, use "an".

An eagle
An apple
An island

Be careful! It is a vowel and consonant sound, not letter.
Think about how these sounds affect the article:

A university
A unicorn
An honest person
An hour

Zero Article

The zero article does not have "a", "an", or "the" before the
noun. Don't use an article in these situations:

Non-specific plural nouns