"A" and "an" are indefinite articles
used before non-specific members
of a group or category of nouns.
The article "a" appears before
singular nouns starting with a
consonant sound
I ate a banana.
The article "an" comes before singular
nouns beginning with a vowel sound.
Give me an apple.
Definite article
"The" is called a definite article.
"The" appears before nouns which are
specific members of a category.
I want to see the movie George Clooney just made.
"The" appears with previously
mentioned nouns.
We went to a movie last night. The movie was called Spy.
We use "The" with unique things.
The sun.
The front door.
The tallest man in the world.
The United States of America.
The Human Kind.
The zero article
In some cases, there is no article before a
noun, this is called the "zero article."
The zero article appears before proper nouns.
Picasso
Mars
PUCE
It also goes with names of places and institutions
which consist of a proper name and another noun.
There is also no article before the names of
days, months, seasons and holidays.
There is no article before geographical names like
Europe, Italy, Tokyo, and Lake Superior.
The "zero article" applies to names of languages
and nationalities, as with Chinese or English.
If you are describing the people of that
country, use the definite article.
The zero article also goes with
names of sports and academic
subjects.
The "a/the switch"
This switch happen when we use the indefinite articles to
introduce something or someone into the context. When the
information becomes familiar, we can use the definite article.
"Once upon a time, there was an old man and an old woman
who lived in a little cottage - first mention - in a vast open field."
"The old woman and the old man were very hungry, so
the old woman decided to bake a gingerbread man."