The definite article is used with specific nouns. The indefinite article is used for non-specific nouns.
The Definite Article
The definite article "the" is used:
1. To refer to something we’ve already mentioned or has already been specified
2. When it’s assumed there’s only one of something
3. Before superlatives and ordinal numbers
4. Before names of monuments or geographic features (oceans, rivers, deserts, etc.)
The Indefinite Article
The indefinite article "a" or "an" is used:
1. When the identity of a singular noun is not known or not important
2. To show that a noun is part of a group
3. To tell someone’s profession
Use “an” when the next word begins with a vowel or silent “h”. Use “a” when the next word begins with a consonant.