Tra từ a - Từ điển Oxford Advanced Learner 8th

a BrE [ə] NAmE [ə] BrE strong form [eɪ] AmE strong form [eɪ] (also an) 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.
1. used before countable or singular nouns referring to people or things that have not already been mentioned
•a man/horse/unit
•an aunt/egg/hour/x-ray
• I can only carry two at a time.
• There's a visitor for you.
•She's a friend of my father's (= one of my father's friends).
2. used before uncountable nouns when these have an adjective in front of them, or phrase following them
•a good knowledge of French
•a sadness that won't go away
3. any; every
• A lion is a dangerous animal.
4. used to show that sb/sth is a member of a group or profession
• Their new car's a BMW.
• She's a Buddhist.
•He's a teacher.
•Is that a Monet (= a painting by Monet)?
5. used in front of two nouns that are seen as a single unit
•a knife and fork
6. used instead of one before some numbers
•A thousand people were there.
7. used when talking about prices, quantities and rates
Syn: ↑per
• They cost 50p a kilo.
• I can type 50 words a minute.
•He was driving at 50 miles an hour.
8. a person like sb
•She's a little Hitler.
9. used before sb's name to show that the speaker does not know the person
•There's a Mrs Green to see you.
10. used before the names of days of the week to talk about one particular day
•She died on a Tuesday.
See also: ↑an

Word Origin:
[a] Middle English: weak form of Old English ān ‘one’.