A / an: the indefinite article

A and an are two forms of the same little word — the indefinite article. You use them before a singular noun phrase when you're talking about one example of something, without identifying exactly which one for your listener: I saw a cat. (one cat — but you're not saying which cat) versus I saw the cat. (a specific cat both you and the listener already have in mind).

Quick shortcut: choose a or an by the sound the next word starts with, not by its spelling.

A or an? It's about sound, not spelling

Use an before a word that starts with a vowel sound, and a before a word that starts with a consonant sound:

• a car · a dog · a house · a book
• an apple · an egg · an idea · an orange · an umbrella

Most of the time this lines up neatly with the letter a word starts with — but not always, which is why the rule is about sound:

So ✅ an hour, never ❌ a hour — and ✅ a university, never ❌ an university, even though university starts with the letter u.

The same sound rule applies to abbreviations said as separate letters: an MBA, because the letter M is pronounced "em," starting with a vowel sound.

When to use a / an

A/an is for talking about one thing when it isn't already known or identified — often because you're mentioning it for the first time:

• I saw a dog in the park. (the listener doesn't know which dog yet)
• She's reading a book. (some book, not a specific one already mentioned)

It's also the natural choice for saying what someone's job or role is, and for describing something as one example of a category:

• He is a doctor.
• This is an interesting question.
• A tomato is a fruit.

If there's an adjective in front of the noun, choose a or an by the sound of the word that comes right after the article — not the noun itself: an old car (not a old car), a big apple (not an big apple).

Once you mention the same thing again, or once both speaker and listener can identify exactly which one you mean, English switches to the instead of a/an — that's the definite article: I saw a dog. The dog was brown.

A / an is only for singular countable nouns

A/an normally goes with a singular noun that can be counted — one book, one idea, one apple. It doesn't go directly before an ordinary plural noun, or before an uncountable noun (like water, money, or advice) in its basic meaning, since those work differently — see Countable & uncountable nouns for the full picture:

• ✅ a book → plural: books (no article, not ❌ a books)
• ✅ an idea → plural: ideas
• ❌ an advice → ✅ some advice / a piece of advice (advice is uncountable, so it has no plural and no a/an of its own)

(You may also hear a water or a coffee in a café, meaning "a glass of water" or "a cup of coffee" — a shortcut that works because everyone understands the missing word.)

Common mistakes

• ❌ a apple → ✅ an apple (apple starts with a vowel sound)
• ❌ an book → ✅ a book (book starts with a consonant sound)
• ❌ a hour → ✅ an hour (the h is silent, so the word starts with a vowel sound)
• ❌ an university → ✅ a university (university starts with a consonant sound, y, despite the vowel letter)
• ❌ She is doctor. → ✅ She is a doctor. (singular jobs and roles almost always need a/an in English)
• ❌ I need an information. → ✅ I need some information. / ✅ I need a piece of information. (information is uncountable, so it never takes a/an and never has a plural -s)

Quick check

Which one sounds right, a or an?

1. I'd like ____ orange, please.
2. She works as ____ engineer.
3. We waited for ____ hour.
4. He's reading ____ university brochure.

Key takeaways

• A and an are the same word — the indefinite article — used for one non-specific, singular, countable thing.
• Choose between them by sound: an before a vowel sound, a before a consonant sound (an hour, a university).
• Use a/an for a first mention, a job or role, or one example of a category; switch to the once it's already identified.
• A/an doesn't go directly with plural or uncountable nouns.