a
indefinite article The form a is used before consonant sounds and the form an before vowel sounds. When saying abbreviations like ‘FM’ or ‘UN’, use a or an according to how the first letter is said. For example, F is a consonant, but begins with the sound /e/ and so you say: an FM radio . U is a vowel but begins with /j/ and so you say: a UN declaration .-
used before countable or singular nouns referring to people or things that have not already been mentioned （用于可数名词或单数名词前，表示未曾提到的）一（人、事、物） -
used to show that somebody/something is a member of a group or profession （表示为某一群体或职业中的一员） Their new car's a BMW. 他们的新轿车是辆宝马。 She's a Buddhist. 她是佛教徒。 He's a teacher. 他是教师。 Is that a Monet (= a painting by Monet)? 那是莫奈的画吗？
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any; every 任何一；每一 -
used before uncountable nouns when these have an adjective in front of them, or phrase following them （用于前有形容词或后有短语的不可数名词前） a good knowledge of French 精通法语 a sadness that won’t go away 挥之不去的悲愁
used in front of two nouns that are seen as a single unit （用于视为一体的两个名词之前） a knife and fork 一副刀叉
used instead of one before some numbers （用于某些数字前，代替 one） used when talking about prices, quantities and rates （用于价格、数量、比率等）一，每一 They cost 50p a kilo. 这些东西一公斤 50 便士。 I can type 50 words a minute. 我每分钟能打 50 个单词。 He was driving at 50 miles an hour. 当时他正以每小时 50 英里的速度驾车。
a person like somebody 像（某人）的人；…式的人物 used before somebody’s name to show that the speaker does not know the person （用于某人姓名前，表示说话者不认识此人）有个 used before the names of days of the week to talk about one particular day （用于一星期中某天的名称前，表示具体某一天）
Word OriginMiddle English: weak form of Old English ān ‘one’.