Grammar: Article "a" or "an"

The articles "a" or "an" are used to express "un" or "una", that's why they are always followed by a noun. 

For instance: 

• It is an orange
• It is a pen

When we have to use a or an?

• We have to use the article "a" when the first sound of the word that follows is a consonant sound.

For instance: a book, a computer, a table, a chair, a window, a pencil case, a hospital...

• We hace to use the article "an" when the first sound of the word that follows is a vowel sound.

For instance: an egg, an apple, an orange juice, an animal, an orange,...