Use of Indefinite Articles

Indefinite Articles are used with Singular Countable Nouns. They are used for the Nouns, but according to the word immediately following them.

Use of Article 'A'

Article 'a' is used with a Noun beginning with a consonant sound; for example : a magazine, a garden, a carpet, a university, a unit, a one-room house.

Note the example a university. In it, 'a' has been used because it is spoken with a Consonant sound (yu) despite its spelling beginning with 'u'.

Food for Thought : Why do you think 'a' has been used with 'unit' and 'one-room flat'?

Use of Article 'An'

Article 'an' is used with a Noun beginning with a vowel sound; for example : an ant, an orange, an egg, an American, an honest man.

Note the example an honest man. The word 'honest' has a Vowel sound though its spelling begins with a Consonant (h).

Indefinite Articles are used for the Nouns, but according to the word immediately following them :

a window               an upper window

a game                   an entertaining game

an egg                    an oval egg

an octopus             a large octopus