As a verb:
* My mother nags me constantly about cleaning my room.
* He nagged her to go to the doctor for weeks.
* Don't nag him about his homework, just let him do it at his own pace.
As a noun:
* She became a real nag after she retired.
* Constant nagging can really wear you down.
* He gave her a withering look and said, "Stop with the nagging!"
In a figurative sense:
* The nagging doubt in the back of my mind wouldn't go away.
* That nagging feeling that something was wrong turned out to be true.
The tone of the sentence can change the meaning of "nag". It can be used in a humorous, negative, or even a sympathetic way depending on the context.