Use of Articles a / an

1. Rule

1.1. A

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

1.2. Some examples are

1.2.1. a cat

1.2.2. a heart

1.2.3. a book

1.2.4. a pencil

2. 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

2.1. Some examples are

2.2. an egg

2.3. an orange

2.4. an arm

3. 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.

4. Rule

4.1. AN

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

4.2. Some examples are

4.2.1. an animal

4.2.2. an apple

4.2.3. an hour

4.2.4. an alligator