The adjectives a, an, and the are called articles.
We use a before a word that begins with a consonant sound.
We use an before a word that begins with a vowel sound.
A and an are usually used to show that there is one of something.

An ape is holding a banana.

A bird is sitting in its nest.
We use the before a word that begins with a consonant or a vowel.
We use the when we are talking about something that is specific or definite.

The children are running in the park.

The baby is in the basket.

The dogs are digging a hole.