Indefinite articles in English are 'a' and 'an.' They are used with singular countable nouns when the noun is not specific. They introduce something for the first time or refer to one member of a group. The choice between 'a' and 'an' depends on the sound that follows, not on spelling.

Use 'a' before words that begin with a consonant sound. Use 'an' before words that begin with a vowel sound. The rule applies to sounds at the start of the word or phrase, including when an adjective comes before the noun.

Rule
Use 'a' before a consonant sound.
Use 'an' before a vowel sound.

The article depends on pronunciation, not spelling. Some words start with a written vowel but have a consonant sound, so they take 'a.' Some words start with a written consonant but have a vowel sound, so they take 'an.'

Rule
Use 'a' before words with a 'y' sound at the beginning.
Use 'an' before words that begin with a short 'e' or 'i' sound, even if the spelling starts with a consonant like 'h.'

'A' is common before words like 'book,' 'cat,' and 'university' because they start with a consonant sound. 'An' is common before words like 'apple,' 'hour,' and 'idea' because they start with a vowel sound. These patterns help predict which article to use in everyday language.

Word/Phrase	Definition
a book	This uses 'a' because 'book' begins with a consonant sound.
a university	This uses 'a' because 'university' begins with a 'y' sound.
an apple	This uses 'an' because 'apple' begins with a vowel sound.
an hour	This uses 'an' because 'hour' begins with an 'ow' sound.
an idea	This uses 'an' because 'idea' begins with a vowel sound.

Use 'a' or 'an' only with singular countable nouns. Do not use 'a' or 'an' with plural nouns or with uncountable nouns. The article introduces one item or one instance in the category.

Rule
Use 'a' or 'an' with singular countable nouns.
Do not use 'a' or 'an' with plural nouns.
Do not use 'a' or 'an' with uncountable nouns.

The choice of 'a' or 'an' depends on the first sound of the next word in the phrase. If an adjective comes before the noun, apply the rule to the adjective. Look for the sound at the beginning of the phrase segment.

Word/Phrase	Definition
an orange book	This uses 'an' because 'orange' begins with a vowel sound.
a big apple	This uses 'a' because 'big' begins with a consonant sound.
an honest person	This uses 'an' because 'honest' begins with an 'o' sound.

'A' is usually pronounced with a short, weak sound, often /ə/. 'An' is pronounced with a short /ən/, and the 'n' links smoothly to the following vowel sound. Clear pronunciation helps listeners understand where one word ends and the next begins.

Word	Notation	Description
a	/ə/	This is the usual unstressed pronunciation in speech.
an	/ən/	The 'n' links to the next word that begins with a vowel sound.