A complete sentence needs a subject and a verb. The verb can show action or can link the subject to a noun or an adjective.
An action verb shows action.
Bob threw the ball.
Janet mowed the lawn.
The verbs threw and mowed both show action.
A linking verb shows no action. Instead, it shows existence and links the subject to something else.
Mary is a girl.
John was sad.
In these sentences, there is no action. Nothing is happening. Mary is existing as a girl. John is existing with sad feelings.
Use a verb in a sentence to show action or existence.
It’s your turn. Find the verb in this sentence. Sammy rode his bike. Does the verb show action or existence?