Articles an/a

1. A

1.1. Used only in singular nouns

1.1.1. a pencil

1.1.2. a book

1.1.3. a clock

1.1.4. a TV

2. An

2.1. Used with words that start with a vowel sound

2.1.1. an star

2.1.2. an house

2.1.3. an wallet

2.1.4. an door