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	🇨🇳 中文 规则	Description	🇨🇳 中文 描述	Notation	🇨🇳 中文 音标	Example	🇨🇳 中文 示例
Use a	🧩 使用 a	Before a consonant sound	🧩 在辅音音素前	/ə/	🧩 /ə/	a dog	🧩 一只狗
Use an	🧩 使用 an	Before a vowel sound	🧩 在元音音素前	/ən/	🧩 /ən/	an apple	🧩 一个苹果
Silent h	🧩 静默的 h	If h is not pronounced, treat the word as starting with a vowel sound	🧩 如果 h 未发音，视该词以元音音素开头	/ən/ + vowel sound	🧩 /ən/ + 元音音素	an hour	🧩 一个小时
Consonant sound from vowel letter	🧩 由元音字母发出的辅音音素	If the word starts with a /j/ sound, use a	🧩 如果单词以 /j/ 的音开头，使用 a	/ə/ + /j/	🧩 /ə/ + /j/	a university	🧩 一所大学
Letter names	🧩 字母名称	With abbreviations, choose by the first spoken sound of the letter name	🧩 使用缩写时，按字母名称的首个发音来决定	an if it starts with /e/ /a/ /i/ /o/	🧩 an 如果以 /e/ /a/ /i/ /o/ 开头	an MBA	🧩 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	🇨🇳 中文 规则	Example	🇨🇳 中文 示例
Singular countable nouns take a/an	🧩 单数可数名词使用 a/an	a chair	🧩 一张椅子
Plural nouns do not take a/an	🧩 复数名词不带 a/an	chairs	🧩 椅子
Use some for plural or uncountable when the meaning is indefinite	🧩 当含义不确定时对复数或不可数使用 some	some chairs, some water	🧩 一些椅子，一些水
Use a measure phrase for uncountable nouns	🧩 对不可数名词使用量词短语	a bottle of water	🧩 一瓶水

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	🇨🇳 中文 规则	Description	🇨🇳 中文 描述	Notation	🇨🇳 中文 音标	Example	🇨🇳 中文 示例
Weak a	🧩 弱读 a	Unstressed a is usually reduced	🧩 非重读的 a 通常被弱化发音	/ə/	🧩 /ə/	I need a pen	🧩 I need a pen
Weak an	🧩 弱读 an	Unstressed an is usually reduced	🧩 非重读的 an 通常被弱化发音	/ən/	🧩 /ən/	I ate an apple	🧩 I ate an apple
Stressed form	🧩 强读形式	Stress to emphasize one or contrast	🧩 强调一个或对比时重读	/eɪ/ or /æn/	🧩 /eɪ/ 或 /æn/	Not two, just a ticket	🧩 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	🇨🇳 中文 词语/短语	Definition	🇨🇳 中文 定义	Example	🇨🇳 中文 示例
once a day	🧩 once a day	one time per day	🧩 one time per day	I take it once a day	🧩 I take it once a day
twice a week	🧩 twice a week	two times per week	🧩 two times per week	We meet twice a week	🧩 We meet twice a week
a few	🧩 a few	a small number of	🧩 a small number of	I have a few questions	🧩 I have a few questions
a lot of	🧩 a lot of	a large amount or number of	🧩 a large amount or number of	She has a lot of experience	🧩 She has a lot of experience
in a hurry	🧩 in a hurry	needing to go quickly	🧩 needing to go quickly	I am in a hurry	🧩 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.