* Verb: The verb is sketch. It's the action word that tells us what the subject is doing.
* Linking: There is no linking verb in this sentence. Linking verbs connect the subject to a noun or adjective that renames or describes it.
Explanation:
"Sketch" is an action verb. It describes a physical act of drawing. The sentence is a command or instruction, asking someone to create a picture. There's no need for a linking verb to connect the subject (implied "you") to any descriptive information.