We use a/an for nouns that we are mentioning for the first time or that are not specific. We use the for specific things.
We use a/an for nouns that are not specific, when we can refer to any one of a certain kind of thing. For example, imagine your uncle just made cookies. You say: "I want a cookie!"
The word “cookie” begins with a consonant sound, so we use a. If the word begins with a vowel sound, we use an. For example: The scientist had an idea.
We use the for specific nouns, when we know exactly which thing we are talking about. We use the for singular or plural nouns.
For Example:
Jack fell asleep in the green chair.
Eric and Adam are coloring the eggs.
We also use the when there is only one of a thing.
For example:
The sun is shining today!
The first time we mention something in a story, we use a/an. After that, we use the.
For Example:
Anna told Rick a secret. The secret was a big surprise! Rick told the secret to Angela later.
Remember that in some cases, we do not use an article. When we are talking about something in general or when we mention a city or country, we do not use an article.
For Example:
Parties are a lot of fun!
I went to Australia last year.