There are many types of verbs, and among them are ‘action’ verbs. In action verbs someone is doing something; grammatically speaking, we call the ‘do-er’ of the action verb the ‘subject’ of the verb. Here are bunch of sentences where you can see the subject of the verb:
John exercises every day.
Karen bought a kilo of plutonium.
David receives 800 e-mail messages every day.
Amelia rejected half a dozen screen plays.
What disrupted the power grid?
Who invented cotton candy?