Is not talking or yelling a sentence?

You're right to think about this! "Not talking" and "yelling" aren't sentences in the traditional sense. Here's why:

* Sentences need a subject and a verb. "Not talking" describes an action, but doesn't tell us *who* is not talking. "Yelling" is the same - we don't know who is doing the yelling.

* Sentences convey a complete thought. "Not talking" or "yelling" only hint at an action. They don't give us information about what is happening or why.

To make them sentences, we need to add a subject:

* He is not talking.

* The children are yelling.

Now we have complete thoughts, with subjects and verbs.