The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns.
the = definite article
a/an = indefinite article
a + singular noun beginning with a consonant: a boy; a car; a bike; a zoo; a dog
The definite article is used before singular and plural nouns when the noun is specific or particular. The signals that the noun is definite, that it refers to a particular member of a group. For example:
“The dog that bit me ran away.” Here, we’re talking about a specific dog, the dog that bit me.