1. How to Use Indefinite Articles
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.
In addition to serving as an article, “a” and “an” are also used to express a proportional relationship, such as "a dollar a day" or "$150 an ounce" or "A sweet 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.
2. Distinction between “A” and “An”
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. However, where the "h" is clearly pronounced, this usage is now less common, and "a" is preferred.
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).
3. "A", "An", "The" and No Article
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? or Would you like any tea?