When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
When it doesn't have a noun ( subject), a verb (the predicate) , no punctuation, when the first letter isn't capitalized and when it hasn't a complete thought. You need all of this to have a complete...
A sentence is a sentence because it has a subject (noun or noun phrase), a predicate ( verb) and punctuation as well as the other parts of speech like pronouns ( she, it him, her, they, them)...