1. Sentence Boundaries: Fragments, Run-Ons & Comma Splices
A sentence-boundary error happens when punctuation fails to mark where one independent clause ends and another begins.
Fragment = missing subject/verb or a lone subordinate clause; run-on = two independent clauses joined with no punctuation; comma splice = two independent clauses joined by only a comma. Fixes: a period, a semicolon, a comma plus a FANBOYS conjunction, or a subordinating conjunction.
A comma splice (comma alone) versus a correctly joined compound sentence (comma + coordinating conjunction such as ``and'' or ``but'').
CLEP often disguises a comma splice as the ``no error'' choice because the sentence reads smoothly aloud --- check the punctuation, not the rhythm.
Two full sentences $arrow$ need a period, semicolon, or comma+FANBOYS, never a comma alone.