Here's why:
* Verbs describe actions or states of being. "Sing" is an action.
* Adjectives describe nouns. They tell us what kind, how many, or which one.
Let's look at an example:
* Sing (verb): The birds sing sweetly.
* Singing (adjective): She has a beautiful singing voice.
In the second example, "singing" is acting as an adjective modifying the noun "voice".