We do not use a / an with names or nouns that are written in plural or be countless, because a / an means "one."

We use a / an when
we describe people or things.

Use "an" when the word start with a vowel.

When the word start with h, use "an"

An hat

An horse

Subtema

With words like 'umbrella', 'ice cream' and 'apple' you have to use: "an umbrella", "an ice cream" and "an Apple".

An Apple

An Orange

A Book

A Computer

A mause

a Test