Indefinite articles in English are the words “a” and “an.” They introduce a singular, countable noun when the listener or reader does not know exactly which one you mean, or when the specific identity is not important. They often appear when you mention something for the first time. They cannot be used with plural nouns or with most uncountable nouns.

Use “a” before a word that begins with a consonant sound, and “an” before a word that begins with a vowel sound. The choice depends on pronunciation, not spelling. This is why some words starting with a vowel letter take “a,” and some words starting with a consonant letter take “an.”

Rule	Description	Notation	Example
Use a	Before a consonant sound	/ə/	a dog
Use an	Before a vowel sound	/ən/	an apple
Silent h	If h is not pronounced, treat the word as starting with a vowel sound	/ən/ + vowel sound	an hour
Consonant sound from vowel letter	If the word starts with a /j/ sound, use a	/ə/ + /j/	a university
Letter names	With abbreviations, choose by the first spoken sound of the letter name	an if it starts with /e/ /a/ /i/ /o/	an MBA

Indefinite articles can mean “one” in a general way: one item, one person, one example. They often suggest that something is not unique or not yet identified. In stories and explanations, they help introduce new people or things before later referring to them with “the” or another specific reference. They can also describe someone’s role or type, not a particular individual.

Use “a/an” when you mention something for the first time because it is new information for the listener. After the first mention, you often switch to “the” once the noun becomes specific in the conversation. This pattern helps the listener track what is new versus what is already known. It is especially common in narratives and instructions.

Use “a/an” to classify something as one example of a group: a city, an animal, a tool, a method. This use focuses on what kind of thing it is, not which exact one. It is common in definitions, descriptions, and when giving someone’s job, identity, or role. With professions, use “a/an” in simple statements about what someone is.

Indefinite articles are only used with singular countable nouns. Do not use “a/an” with plural countable nouns; instead use a plural form with no article or use a quantifier like “some.” Do not use “a/an” with uncountable nouns; use no article, or use a measure phrase like “a piece of” or “a glass of.” These limits are about how English treats countability, not about size or amount.

Rule	Example
Singular countable nouns take a/an	a chair
Plural nouns do not take a/an	chairs
Use some for plural or uncountable when the meaning is indefinite	some chairs, some water
Use a measure phrase for uncountable nouns	a bottle of water

When an adjective comes before the noun, place “a/an” before the adjective, not directly before the noun. Choose “a” or “an” based on the first sound of the adjective, because that is what is pronounced immediately after the article. This is why the same noun can take “a” or “an” depending on the adjective that comes first. This rule also applies when there are multiple adjectives.

In natural speech, “a” and “an” often have weak pronunciations. “A” is commonly pronounced /ə/ and “an” as /ən/ when they are unstressed. They can be stressed to emphasize “one” or to contrast choices, and then they sound clearer and longer. Understanding weak forms helps listening and helps your own speech sound natural.

Rule	Description	Notation	Example
Weak a	Unstressed a is usually reduced	/ə/	I need a pen
Weak an	Unstressed an is usually reduced	/ən/	I ate an apple
Stressed form	Stress to emphasize one or contrast	/eɪ/ or /æn/	Not two, just a ticket

Some common expressions use “a/an” in set patterns where the article feels idiomatic. This includes frequency phrases, certain quantity phrases, and a few time expressions. In these cases, “a/an” helps build a meaning like “per” or “one at a time.” Learning these as chunks improves fluency.

Word/Phrase	Definition	Example
once a day	one time per day	I take it once a day
twice a week	two times per week	We meet twice a week
a few	a small number of	I have a few questions
a lot of	a large amount or number of	She has a lot of experience
in a hurry	needing to go quickly	I am in a hurry

Use “a/an” to talk about one singular countable thing in a general or newly introduced way. Choose “a” or “an” by the first sound that follows, not by spelling. Avoid “a/an” with plural or uncountable nouns, and use “some” or measure phrases instead. These rules let you sound natural and make your meaning clear in both writing and speech.