The definite article the is used when the referent of the noun phrase is assumed to be unique or known from the context. For example, in the sentence The boy with glasses was looking at the moon, it is assumed that in the context the reference can only be to one boy and one moon. However, the definite article is not used:
with generic nouns (plural or uncountable): cars have accelerators, happiness is contagious, referring to cars in general and happiness in general (compare the happiness I felt yesterday, specifying particular happiness);
with many proper names: John, France, London, etc.
The indefinite article a (before a consonant sound) or an (before a vowel sound) is used only with singular, countable nouns. It indicates that the referent of the noun phrase is one unspecified member of a class. For example, the sentence An ugly man was smoking a pipe does not refer to any specifically known ugly man or pipe.
No article is used with plural or uncountable nouns when the referent is indefinite (just as in the generic definite case described above). However, in such situations, the determiner some is often added (or any in negative contexts and in many questions). For example:
There are apples in the kitchen or There are some apples in the kitchen;
We do not have information or We do not have any information;
Would you like tea? or Would you like some tea? and Would you like any tea? or Would you like some good tea?
Additionally, articles are not normally used:
in noun phrases that contain other determiners (my house, this cat, America's history), although one can combine articles with certain other determiners, as in the many issues, such a child.
with pronouns (he, nobody), although again certain combinations are possible (as the one, the many, the few).
preceding noun phrases consisting of a clause or infinitive phrase (what you've done is very good, to surrender is to die).
If it is required to be concise, e.g. in headlines, signs, labels, and notes, articles are often omitted along with certain other function words. For example, rather than The mayor was attacked, a newspaper headline might say just Mayor attacked.
For more information on article usage, see the sections Definite article § Notes and § Indefinite article below. For more cases where no article is used, see Zero article in English.
In most cases, the article is the first word of its noun phrase, preceding all other adjectives and modifiers.
The little old red bag held a very big surprise.
There are a few exceptions, however:
Certain determiners, such as all, both, half, double, precede the definite article when used in combination (all the team, both the girls, half the time, double the amount).
The determiner such and exclamative what precede the indefinite article (such an idiot, what a day!).
Adjectives qualified by too, so, as and how generally precede the indefinite article: too great a loss, so hard a problem, as delicious an apple as I have ever tasted, I know how pretty a girl she is.
When adjectives are qualified by quite (particularly when it means "fairly"), the word quite (but not the adjective itself) often precedes the indefinite article: quite a long letter.
The only definite article in English is the word the, denoting person(s) or thing(s) already mentioned, under discussion, implied, or otherwise presumed familiar to the listener or reader. The is the most commonly used word in the English language, accounting for 7% of all words.
"The" can be used with both singular and plural nouns, with nouns of any gender, and with nouns that start with any letter. This is different from many other languages which have different articles for different genders and/or numbers.
The indefinite article of English takes the two forms a and an. Semantically, they can be regarded as meaning "one", usually without emphasis. They can be used only with singular countable nouns.
The form an is used before words starting with a vowel sound, regardless of whether the word begins with a vowel letter. This avoids the glottal stop (momentary silent pause) that would otherwise be required between a and a following vowel sound. Where the next word begins with a consonant sound, a is used. Examples: a box;an apple;an SSO (pronounced "es-es-oh"); a HEPA filter (HEPA is pronounced as a word rather than as letters); an hour (the h is silent); a one-armed bandit (pronounced "won..."); an heir (pronounced "air"); a unicorn (pronounced "yoo-"); an herb in American English (where the h is silent), but a herb in British English.
Some speakers and writers use an before a word beginning with the sound /h/ in an unstressed syllable: an historical novel, an hotel. When this is spoken, the "h" is not pronounced so "an hotel", for example, sounds like "an 'otel". However, this usage is now less common.
Some dialects, particularly in England (such as Cockney), silence many or all initial h sounds (h-dropping), and so employ an in situations where it would not be used in the standard language, like an 'elmet (standard English: a helmet).
There used to be a distinction analogous to that between a and an for the possessive determiners my and thy, which became mine and thine before a vowel, as in mine eyes.