Question

When can a verb be called a sentence by itself?

Answer 1

When it is used with a subject because every sentence needs a subject and verb. For example: I ran.
~JZ
Hope it helps

Answer 2

Sentence structure is a means by which words are organized grammatically so that they can communicate an idea clearly to a reader or listener. Depending on the language that is being used, structure may vary. However, in English, the structure of a sentence is built in such a way that there will always be a minimum set of requirements to meet correct structure criteria. Among these requirements stands the use of subjects and verbs, among other words, to communicate an idea. A verb by itself is just an action, but aside from that, it does not communicate much else. But when this verb is used in conjunction with a Subject, meaning nouns, the verb by itself acquires the capacity to communicate a clear idea. For example: Cristina runs all the time. Runs, would have no meaning by itself unless it was joined by the subject, being Cristina. This is why the correct answer is B, when it is used with a subject.