The indefinite articles a and an introduce a singular noun when the person or thing is not specific. They point to one member of a group, not a particular one that the listener already knows. In I saw a dog, the listener knows there was one dog, but not which dog. If you change the article to a specific form like the, the meaning changes to one known or identified person or thing. Without a or an, the noun often sounds incomplete when you want to name one countable item.

A before consonant sound patterns

Word	Notation	Description	Example
book	b	Use a before a word that begins with a consonant sound.	I read a book last night.
house	h	Use a before a word that begins with a consonant sound even when the spelling is simple.	She lives in a house near the park.
user	j	Use a before a word that begins with a consonant sound even if the first letter is a vowel.	He is a user of the app.

Use a before a word that begins with a consonant sound: a car, a student, a house. The sound matters more than the first letter. So a university is correct because university begins with the y sound. The same pattern appears in phrases like a one-way street, because one begins with the w sound.

An before vowel sound patterns

Word	Notation	Description	Example
apple	æ	Use an before a word that begins with a vowel sound.	I ate an apple.
idea	aɪ	Use an before a word that begins with a vowel sound even when the spelling starts with a consonant letter.	That is an idea I like.
honest	ɒ	Use an before a word with a silent h and a vowel sound.	She gave me an honest answer.

Use an before a word that begins with a vowel sound: an apple, an umbrella, an old friend. The spelling can be misleading, so listen to the first sound. Write an hour because the h is silent, and the word begins with the vowel sound ow. In speech, the article joins smoothly to the next word: an idea, an empty room.

Singular countable noun requirement

Example	Pattern
He bought a sandwich.	Use a or an with singular nouns when the noun names one item.
She saw dogs in the yard.	Do not use a or an with plural nouns.
We need water for the trip.	Do not use a or an with uncountable nouns.

A and an go with singular, countable nouns. Say a chair, an answer, a meal. Do not use them with plural nouns or uncountable nouns. Not a chairs or an information. For plural or uncountable meanings, English often uses no article, or it uses another determiner such as some.

Article position before noun phrases

Usage	Explanation	Example
Basic noun phrase	Use the article directly before the noun in a simple noun phrase.	I found a coin on the floor.
Modified noun phrase	Use the article before any adjectives that come before the noun.	She adopted an old cat.
Complete noun phrase	Use the article at the start of the full noun phrase when the noun has extra words after it.	He wants a cup of tea.

Place a or an directly before the noun phrase. The article comes before adjectives, too: a red car, an interesting book, a very old tree. The pattern is a/an + adjective + noun. If there are several adjectives, the article still stays first: an expensive new phone.

Possessive determiner replacement

Example	Pattern
This is my pen.	Use a possessive determiner instead of an article before a noun.
That is our new house.	A possessive determiner can come before an adjective and a noun.
She opened her notebook.	Do not use a or an before possessive determiners.

Do not use a or an before possessive determiners. Use my, your, his, her, our, or their instead of an indefinite article: my bag, her brother, their house. A possessive determiner already shows which person the noun belongs to, so another article is not possible. The same position is used by other determiners in the noun phrase, including words covered in Determinants.

Article choice in sound exceptions

Word	Notation	Description	Example
hour	aʊ	Use an before words with a silent h and a vowel sound.	It will take an hour.
honest	ɒ	Use an before words where the first sound is a vowel sound even if the spelling begins with h.	He gave an honest reply.
university	ju	Use a before words that begin with a consonant sound like y.	She goes to a university.

Some words begin with a silent consonant letter, so the article follows the first sound, not the spelling. Say an honest person, an hour, an heir. Some words begin with a vowel letter but a consonant sound, so use a: a university, a European city, a one-bedroom flat. In each case, the first spoken sound decides the article.

First mention and one example meaning

Usage	Explanation	Example
First mention	Use a or an when you introduce something for the first time.	I saw a bird in the tree.
Any one example	Use a or an to mean one example of a thing, not a specific one.	I need a phone for work.
Non specific person	Use a or an when the person is not named or known.	We met a doctor at the clinic.

Indefinite articles often appear when you mention something for the first time: I met a doctor, She bought a laptop. They also mean one example from a larger group: I need a taxi means any taxi, not one known in advance. After the first mention, English often switches to the when the speaker and listener now know which person or thing is meant. In some general statements, English uses the zero article instead of a or an, especially with plural and uncountable nouns.