The

The” is a definite article because it is used when the noun is specific or particular (not “any lake,” but “the lake”), while “a/an” are the indefinite articles (“a lake,” meaning “any lake” not a specific lake).

When to use "the" is often the most difficult to figure out because it depends on the structure of the rest of the sentence.
For example, both of the following sentences are correct:

1. Three houses on my block have fenced-in yards.
2. The three houses on my block with fenced-in yards are blue.