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).
The town crier promulgates news in a town. Promulgate means to proclaim so just about any sentence that you can think of with th word "proclaim" in it, can work by just replacing the word "proclaim"...
A sentence is a complete clause with a subject and a finite verb. A sentence fragment is not a complete clause, but is part of a clause (often lacking the verb to make a full sentence). For instance,...