called the indefinite article (or, by some grammarians, determiner). In origin, a and its by-form an are versions of the Old English an meaning ‘one’.
1 Before all normal words or diphthongs an is required (an actor, an eagle, an illness, an Old Master, an uncle). Before a syllable beginning in its written form with a vowel but pronounced with a consonantal sound, a is used (a eulogy, a unit, a use; a one, a once-only). Before all consonants except silent h, a is usual: a book, a history, a home, a household name, a memorial service, a puddle, a young man; but, with silent h, an hour, an honour.
2 In most circumstances a is pronounced with an unstressed indeterminate sound ǝ or ǝn , but it is sometimes emphasized as ay or an in slow diction or to emphasize singleness (I said a piece, not several). Practice differs with h-words in which the first syllable is unstressed: a (or an) habitual criminal; a (or an) hotel. There is evidence, especially in written English, for the continued use of an before habitual, historian, historic(al), horrific, and horrendous, but the choice of form remains open. However, an hotel now sounds dated (1930s) and a hotel is more usual.
3 With single letters and groups of letters that are pronounced as individual letters, be guided by the pronunciation: a B road, a TUC leader; but an A road, an FA Cup match, an SAS unit (assuming the abbreviations are not mentally expanded to their full forms, which would alter the choice).
4 A and an normally precede the word or words they determine (a popular person, an ugly building), but it follows the adjectives many, such, and what (many a year, such a family, what an awful nuisance!). It also follows any adjective preceded by as or how (Iris Murdoch is as good a writer as Virginia Woolf / He did not realize how tiresome a person he could be) and often an adjective preceded by so (So bold a move deserved success), although such is now more usual (Such a bold move deserved success). In some circumstances the positioning is optional: either before or after an adjective preceded by too (too strict a regime or a too strict regime) and before or after the adverbs quite and rather (at quite an early hour or at a quite early hour; it's rather a hard puzzle or it's a rather hard puzzle). With few and lot, however, the only possible order is quite a few and quite a lot. A good few is now commonly used.
5 A and an are also used to distinguish a particular person or artistic or literary creation: Do you know a Lucy Smith? / They own a Van Gogh / She plays a Broadwood [piano]; and to denote a standard quantity of something that is normally uncountable: Do you want a beer? / I've been trying a new cheese. Note also the following uses in time measurement: once a fortnight, £20,000 a year, half an hour, 50 miles an hour.
6 The indefinite article has been used with nouns of multitude (a dozen eggs, a million pounds) for centuries. A fairly recent extension of this use is with an adjective (usually the present or past participle of a verb) between article and noun: The police found themselves confronted by an estimated two hundred youths / The dyke was an astonishing 30 feet wide.