a

First recorded in 1200–50; Middle English; originally preconsonantal phonetic variant of an¹

In both spoken and written English the choice of a1 or an¹ is determined by the initial sound of the word that follows. Before a consonant sound, a is used; before a vowel sound, an : a book, a rose; an apple, an opera. Problems arise occasionally when the following word begins with a vowel letter but actually starts with a consonant sound, or vice versa. Some words beginning with the vowel letter u and all words beginning with the vowel letters eu are pronounced with a beginning consonant sound, as if the first letter were y : a union; a European. Some other spellings that begin with a vowel letter may also stand for an initial consonant sound: a ewe; a ewer. The words one and once and all compounds of which they are the first element begin with a w sound: a one-room apartment; a once-famous actor.
The names of the consonant letters f, h, l, m, n, r, s, and x are pronounced with a beginning vowel sound. When these letters are used as words or to form words, they are preceded by an : to rent an L-shaped studio; to fly an SST. The names of the vowel letter u and the semivowel letters w and y are pronounced with a beginning consonant sound. When used as words, they are preceded by a : a U-turn; The plumber installed a Y in the line.
In some words beginning with the letter h, the h is not pronounced; the words actually begin with a vowel sound: an hour; an honor. When the h is strongly pronounced, as in a stressed syllable at the beginning of a word, it is preceded by a : a history of the Sioux; a hero sandwich. (In former times an was used before strongly pronounced h in a stressed first syllable: an hundred. ) Such adjectives as historic, historical, heroic, and habitual, which begin with an unstressed syllable and often with a silent or weakly pronounced h, are commonly preceded by an, especially in British English. But the use of a rather than an is widespread in both speech and writing: a historical novel; a habitual criminal. Hotel and unique are occasionally preceded by an, but this use is increasingly old-fashioned. Although in some dialects an has yielded to a in all cases, edited writing reflects usage as described above.

Originally Middle English a, preconsonantal variant of on (see a-¹); confused with a¹

Middle English; unstressed preconsonantal variant of of¹

First recorded in 1300–50; Middle English; a phonetic variant of have

First recorded in 1200–50; Middle English a, ha

Middle English, late Old English; cf. a², nowadays

Middle English; see a³

Middle English; Old English a- (unstressed), ǣ-, ā-, ō- (stressed; see abb, woof¹, oakum), rarely or- (see ordeal), ultimately from unattested Germanic uz- from unstressed Indo-European uss-, from ud-s (these latter two also unattested), akin to out; in some cases confused with a-⁴, as in abridge

Middle English <Latin ā-, a- (variant of ab-ab-); in some words <French a-<Latin ab-, as in abridge

Middle English, in some words <Middle French a-<Latin ad- prefix or ad preposition (see ad-), as in abut; in others <Latin a- (variant of ad-ad-), as in ascend

From Latin feminine ending -a, as Claudia, feminine of Claudius; see -a²

Probably generalized from the -a of magnesia

<Latin annō,ablative ofannus

From the Latin word ante

<Latin annō, ablative of annus

From the Latin word ante