Use A and An.

1. It is used "an" when the word begins with a vowel sound. example: an apple an orange an eraser.

1.1. an earth.

2. Video.

3. an hour.

4. exceptions: There are words beginning with a consonant but the first sound of the word is a vowel sound. example: an hour (do not pronounce the "h" in the word.)

5. The words "ay" an "are articles to indicate one thing (like" a "or" one "in Spanish). Example: a book a man a game

5.1. A book.