Use of Articles a / an

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. Rule

2.1. A

2.1.1. Use “a”, “and” only with singular nouns or if you start with a consonant

2.2. Some examples are

2.2.1. a cat

2.2.2. a heart

2.2.3. a book

2.2.4. a pencil

3. Rule

3.1. AN

3.1.1. Use “an” with words that start with a vowel sound or h mute

3.2. Some examples are

3.2.1. an animal

3.2.2. an apple

3.2.3. an hour

3.2.4. an alligator

4. Although there is an important exception where the indefinite article "a", the indefinite article "a" becomes "an " is used if the following noun begins with a vowel

4.1. Some examples are

4.2. an egg

4.3. an orange

4.4. an arm