a
the letter A
Letter name · 字母名称the 1st letter of the English alphabet
英文字母 A（第 1 个字母）
Source: BridgeDict Latin alphabet catalog
一个；某一
先理解核心含义，再看它在真实句子里的用法。
一个；某一
Used before a singular countable noun to refer to a nonspecific or not previously mentioned person or thing.
She bought a book yesterday.
她昨天买了一本书。
The noun sense means the letter itself. Scientific meanings appear only when their capitalization exactly matches your query.
the 1st letter of the English alphabet
英文字母 A（第 1 个字母）
Source: BridgeDict Latin alphabet catalog
the SI prefix atto, representing a factor of 10^-18
国际单位制词头 atto，表示 10^-18
Source: BIPM SI BrochureI saw a dog in the park.
我在公园里看到一只狗。
She wants to buy a car next year.
她想明年买一辆车。
He is reading a book about history.
他正在读一本关于历史的书。
用于首次提及或非特指的单数可数名词前。
She bought a book yesterday.
她昨天买了一本书。
这里 a 表示“每”，相当于 per。
We have a meeting once a week.
我们每周开一次会。
此处 a 强调数量一，相当于 one。
He ran a mile in six minutes.
他六分钟跑了一英里。
Not this
She is teacher.
Use this
She is a teacher.
可数名词单数前不能缺少限定词；teacher 是可数名词单数，必须用 a 表示“一位”。
Not this
I have a information.
Use this
I have information.
information 是不可数名词，不能用 a 修饰；表示泛指时前面不加冠词。
Not this
He is a honest man.
Use this
He is an honest man.
honest 虽然以 h 开头，但 h 不发音，实际以元音音素开头，所以要用 an 而不是 a。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
许多；大量
She has a lot of friends.
她有很多朋友。
几个；一些
I'll be back in a few minutes.
我几分钟后回来。
一点；少量
There is a little milk left.
还剩一点儿牛奶。
每周一次
I go swimming once a week.
我每周去游泳一次。
两个；几个
We stayed there for a couple of days.
我们在那里待了两三天。
每次；一次
Please lift only one box at a time.
请一次只搬一个箱子。
小菜一碟；轻而易举的事
The math test was a piece of cake.
数学考试是小菜一碟。
多得很；不稀罕
In this city, coffee shops are a dime a dozen.
在这个城市，咖啡店到处都是。
因祸得福；塞翁失马
Missing the train was a blessing in disguise.
错过了火车反倒是因祸得福。
千里之行，始于足下。
She took a small step toward her dream.
她朝着自己的梦想迈出了一小步。
玫瑰即使换个名字，依然芬芳。
He gave her a red rose.
他送给她一朵红玫瑰。
Use a naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
把字母a想象成一根横放的“1”，读音像“诶”，记：一个a，每单位也是a。
语域：neutral 常见场景：日常会话、正式书面语 a 作为不定冠词，口语和书面语中都很常见，通常用于非特指的单数可数名词前；表示“一个”或“每”的用法也多采用同一形式。
这些词的重读音节都含 /eɪ/，与 a 单独读时的 /eɪ/ 押韵。注意：以 -a 结尾的常见词常读 /ə/（如 data、pasta、banana），不要按拼写读成 /eɪ/；冠词 a 在句中通常弱读为 /ə/。
Dictionary facts from AI_FALLBACK, ECDICT, WIKTIONARY, BridgeDict Latin alphabet catalog, BIPM SI Brochure.