Sentence Fragments

Every real sentence needs a subject and a finite verb — a verb form that actually shows tense, like 'burned' or 'closed,' not a bare -ing or to- form. Take 'Which is why the bridge closed.' It has a subject, 'which,' and a verb, 'is.' But 'which' is a relative pronoun, and relative pronouns point backward to something earlier. They never launch a sentence on their own. Read it alone and you wait for the rest of the thought that never arrives. That's the test: subject, finite verb, and no leftover word up front still asking to be connected to something else.

Short word groups aren't automatically fragments. 'Stop.' works because the subject 'you' is understood, and 'stop' is a finite command verb — the sentence is complete even though it's one word. Length has nothing to do with it; completeness does. The same logic rules out -ing words as sentence verbs. 'The lanterns burning along the wall' looks busy, but 'burning' has no tense by itself. It needs a helper: 'were burning,' or a plain tensed verb like 'burned.' Without one, the group just describes the lanterns instead of reporting an event.

Fragments connect to the rest of sentence-structure work because fixing one usually means choosing how to join clauses. You can give the fragment its own finite verb and punctuate it as a full sentence, or you can attach it to the sentence next to it with a comma, letting that sentence supply the missing subject and verb. 'The council met for three hours, debating the same three amendments the whole time' works because 'debating' now describes the council's action within an already-complete sentence.