Article "an"/"a"

1. Article "a"

1.1. and article "a" is used when the next word starts with a consonant

1.1.1. For example

1.1.1.1. A book

2. Article "an"

2.1. the article "a" becomes "an" when the next word begins with a vocal sound

2.1.1. For example

2.1.1.1. An old book