A and an are called articles.  They are special adjectives.
We use a before words that begin with a consonant sound.

soccer

A puppy is playing soccer.

banana

The monkey is eating a banana.

train

A train is running on the railroads.

We use an before words that begin with a vowel: a, e, i, o, and u.

apple

The mouse is eating an apple.

rain

We use an umbrella in the rain.

igloo

I built an igloo yesterday.

Exercises

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