A and an are called articles. They are special adjectives.
We use a before words that begin with a consonant sound.
A puppy is playing soccer.
The monkey is eating a banana.
A train is running on the railroads.
We use an before words that begin with a vowel: a, e, i, o, and u.
The mouse is eating an apple.
We use an umbrella in the rain.
I built an igloo yesterday.