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I saw a dog in the park.

Used to refer to a single item or one of a kind.
I need a pencil.
She has a cat.
He bought a book.
a car
a friend
a house
one
single
individual
I saw a bird in the tree.

It is an indefinite article used to refer to a non-specific item or person.
She wants a new dress.
He has a dog.
They found a coin.
a lot
a few
a couple
one
any
single