Sentence Structure
A sentence must stand on its own
A complete sentence needs a subject, a finite verb, and a finished thought. Most sentence-structure questions ask you either to find a boundary that is missing (a run-on) or to remove a boundary that should not be there (a fragment).
Fragments usually appear as a dependent clause or a verbal phrase wearing a capital letter: Because the invoice arrived late. Ask "who or what does what?" If no word actually performs an action, add a subject and verb or attach the fragment to the sentence beside it.
Run-ons come in two forms. A fused sentence jams two independent clauses together with no punctuation; a comma splice joins them with only a comma: The shipment arrived Monday, we unpacked it immediately. Fix it three ways: a period, a semicolon, or a comma plus a coordinating conjunction (and, but, or, nor, for, so, yet). A conjunctive adverb such as however or therefore is not a conjunction, so it needs a semicolon or period before it.
Watch parallelism: items in a list or on either side of and must share grammatical form. She drafts reports, schedules meetings, and answers calls - not answering calls. Misplaced and dangling modifiers are the other common trap: an introductory phrase must describe the subject that follows. After reviewing the file, the error was obvious implies the error did the reviewing.
Rule of thumb: cover the conjunction and read each half aloud. If either half could stand alone as a sentence, you need a semicolon, a period, or a comma plus a true conjunction - never a comma by itself.