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!(You want any one of the cookies he has made, not a specific cookie. So, you use a.)
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 do not know anything about the idea, so it is not specific. The word “idea” starts with a vowel sound, so we use an.)
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.
(We know exactly which specific chair we are talking about.)
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!
(There is only one sun.)
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.
(We use a the first time we mention the secret. After that, we use the because we know exactly which specific secret we are talking about.)
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!(Parties in general are fun.)
I went to Australia last year.(Australia is a country, so we do not use an article.)
A preposition is a word that tells us about the relation of one thing to another or the position of things.
Common examples of prepositions are: in, on, at, above, under, below, from, to, by, with, to, for etc.