'A' and 'an' function as the indefinite forms of the grammatical article in the English language and can also represent the number one. English is a West Germanic language originating in England and is the First language for most people in the United Kingdom, the United States An is the older form (related to one, cognate to German ein; etc), now used before words starting with a vowel sound, regardless of whether the word begins with a vowel letter. The German language (de ''Deutsch'') is a West Germanic language and one of the world's major languages. In Phonetics, a vowel is a Sound in spoken Language, such as English ah! or oh!, pronounced with an open Vocal tract Examples: a light-water reactor; an LWR; a sanitary sewer overflow; an SSO; a HEPA filter (because HEPA is pronounced as a word rather than as letters); a hypothesis; an hour; a ewe; a hero; a one-armed bandit; an heir; a unicorn.
In a process called juncture loss, the n has wandered back and forth between words beginning with vowels over the history of the language, where sometimes it would be a nuncle and is now an uncle. Juncture loss (also known as junctural metanalysis, false splitting, misdivision, refactorization, or rebracketing) is the The Oxford English Dictionary gives such examples as smot hym on the hede with a nege tool from 1448 for smote him on the head with an edge tool and a nox for an ox and a napple for an apple. The Oxford English Dictionary ( OED) published by the Oxford University Press (OUP is a comprehensive Dictionary of the English Sometimes the change has been permanent. For example, a newt was once an ewt (earlier euft and eft), a nickname was once an eke-name, where eke means "extra" (as in eke out meaning "add to"), and in the other direction, "a napron" became "an apron" and "a naddre" became "an adder. A newt is an Amphibian of the Salamandridae family order Urodela or Caudata, found in North America, Europe, and " "Napron" itself meant "little tablecloth" and is related to the word "napkin". An oft-cited but inaccurate example is an orange: despite what is often claimed, English never used a norange. Although the initial n was in fact lost through juncture loss, this happened before the word was borrowed in English (see orange (word)). Orange is both a Noun and an Adjective in the English language.
The form "an" is always prescribed before words beginning with a silent h, such as "honorable", "heir", "hour", and, in American English, "herb". Some British dialects (for example, Cockney) silence all initial h's (h-dropping) and so employ "an" all the time: e. The term Cockney has both geographical and linguistic associations The phonological history of English fricatives and affricates is part of the Phonological history of the English language in terms of changes in the Phonology of g. , "an 'elmet". Many British usage books, therefore, discount a usage which some Americans (amongst others) employ as being a derivative of the Cockney. The reason is that the indefinite article a is pronounced either of two ways: as a schwa, or as the letter itself is pronounced, "long a" (actually a diphthong, /eɪ/). In Linguistics, specifically Phonetics and Phonology, schwa can mean the following An unstressed and toneless neutral Some words beginning with the letter h have the primary stress on the second or later syllable. Pronouncing a as a schwa can diminish the sound of the schwa and melt into the vowel. Pronouncing it as a "long a" does not do this, but as the pronunciation cannot be prescribed, the word is spelled the same for either. Hence an may be seen in such phrases as "an historic", "an heroic", or "an hôtel of excellence", which was the by-line in an advertisement in a New York City newspaper.
Merriam-Webster's Dictionary of English Usage is more descriptive than prescriptive, but it advises, "You choose the article that suits your own pronunciation. " Theodore Bernstein gives the straight vowel-sound vs. consonant-sound explanation but allows that one should indeed say "an hotel" if they think hotel is pronounced otel. 
Such was also the case for some other words which take the place of the article. "My" and "thy" became "mine" and "thine", as in "mine uncle". This usage is now obsolete.
The appearance of an or a in front of words beginning with h is not limited to stress. Sometimes there are historical roots as well. Words that may have had a route into English via French (where all h's are unpronounced) may have an to avoid an unusual pronunciation. Words that derived from German however would use a as the h's would be pronounced. There is even some suggestion that fashion may have had some influence. When England was ruled by a French aristocracy, the tradition may have been to exclusively use an, while when Britain was governed by a German-based monarchy the tide may have changed to a.
Further, some words starting with vowels may have a preceding a because they are pronounced as if beginning with an initial consonant. "Ewe" and "user" have a preceding a because they are pronounced with an initial y consonant sound. "One-armed bandit" and "one-sided argument" also have a preceding a because they are pronounced with an initial w consonant sound.
To add emphasis to a noun, the preceding indefinite article is often pronounced as a long a (just as the definite article would be pronounced as "thee" in such cases), whether or not the schwa, or even "an" would be the appropriate usage. In recent years, there has been a noticeable trend to pronouncing almost all indefinite articles in this way, especially in radio or television announcements or news-reading. This article is about radio broadcasting for other uses see Radio (disambiguation. For the band see Broadcast (band Broadcasting is the distribution of audio and/or Video signals which transmit An announcer is a Voice actor who works in Television, Radio or Film, usually providing Narrations News updates Station
In addition to serving as an article, a and an are also used as synonyms for the number one, as in "make a wish", "a hundred". An was originally an unstressed form of the number án 'one'.
A and an are also used to express a proportional relationship, such as "a dollar a day" or "$150 an ounce" or "A Mars a day helps you work, rest and play", although historically this use of "a" and "an" does not come from the same word as the articles.
The mathematically-minded might heed H. S. Wall's reminder that the statement "I have a son" does not necessarily imply that "I have exactly one son" or that "I have only sons". In other words, "The little words count. " — H. S. Wall, Creative Mathematics.