Theory:

Determiners - Articles - The, A, An

An important specific determiner is the definite article THE, which is the most common word in English.

The definite article the is the most frequent word in English, and is used when the other person knows what we mean, for several reasons:

Don't look at the sun.

I live near the church.

I want the green one.

THE is used for supelative adjectives to identify a unique element:

Joe is the tallest.

THE is used to refer to musical instruments:

He plays the flute.

THE is used to describe services:

You can depend on the police.

We use THE when talking about countries that are states, republics or kingdoms, or have plural names:

I visited the United States, The United kingdom, and the Philippines.

Another common general determiner is the indefinite article A, which is used when the other person doesn't know which item we mean:

I met a girl on the train.

A can only be used for singular countable nouns that don't begin with a vowel sounds, when AN is used instead:

I bought a book yesterday.

I saw an octopus in the sea.

A is used to describe what someone does:

He's a dentist.

THE can be used for non countable nouns, but A and AN can't:

I enjoyed the music.

I went to a concert.

I saw her at an exhibition.