A and An are used when referring to a noun that is not known to the speaker, listener and/or reader.
They are what we call indefinite articles.
A - is used before a word with a consonant SOUND.
Example:
1. A car is parked in front of the house.
2. A ukulele is a string instrument.
An- is used before a word that starts with a vowel SOUND.
Example:
1. An apple is in the box.
2. An hour has 60 minutes.
The, on the other hand, is a definite article and is used before a noun that is already known to the speaker, wrtieer, listener, and/or reader.
Example:
1. The house I live in is right around the corner.
2. The car he drives is red and small.