The Chief Rules - I - Indefinite Articles.
The following are the chief rules as to the use of the articles.
1. 'An' is used when the following word begins with a vowel sound.
*E.g.: An apple, an idle girl, an oval mirror …
Note: For abbreviations, use either "a" or "an" depending on the pronunciation-sound of the first letter in the abbreviation. Such as, "a CV", "a PC” … or "an HTML", "an MP" ... (Not "a HTML", "a MP", ...)
2. ‘An’ is used before Heir, Honour (Honor), Honourable (Honorable), Honest, and Hour, as in these words the H is silent.
E.g.: An hour .... An honest boy ...
3. ‘A’ is used before words beginning with O with the sound of W.
E.g.: A one legged man, a one rupee-four-anna tie, …
4. Also before word beginning with a Y sound but spelt (spelled) with U or EW.
E.g. A union of states, a university, a used stamp, a useful plan, a ewe, …
5. ‘A / An’ is used with a proper noun which has the meaning of a common noun.
E.g.: He is an Akbar and a Napoleon in one.
[*E.g. = Exempli gratia (Latin) = “For example, as a case in point, as an example, as an illustration, for instance, say” | “Conventions of Written English An ABBREVIATION meaning “for example.” It is short for the LATIN exempli gratia, “for the sake of example.” A list of examples may be preceded by e.g.: “She loved exotic fruit, e.g., mangoes, passion fruit, and papayas.”]