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)...
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,...