1.1. According to the Oxford Dictionaries it is a determiner that introduces a noun phrase. a|an are considered indefinite articles and they are used to introduce new concepts into a discourse.
2. Examples
2.1. 1. Use “a”, “and” only with singular nouns 2. Use “an” with words that start with a vowel sound
3. Ruler.
3.1. When you ask a student how to use a or an he will tell you that this depends on whether the word that follows begins with a vowel or a consonant, however that is a common mistake. Actually the use does not depend on the first letter of the word that follows it, but on the first sound of the word that follows it. If the first sound of the word is a vowel then an will be used.
4. Examples
4.1. 1. Use “a”, “and” only with singular nouns 2. Use “an” with words that start with a vowel sound
