Conjunctive Adverbs and the Splice They Hide

Coordinating conjunctions like 'and', 'but', and 'so' are built to join. Put one between two complete sentences with a comma in front, and the comma plus the conjunction together do the joining work: 'The samples thawed, but the DNA survived.' Conjunctive adverbs like 'however', 'therefore', and 'moreover' look like they do the same job because they sit in the same spot and carry similar meaning. They don't. Grammatically they're just adverbs describing the sentence they're attached to, with no power to attach one clause to another. That means the clause before one still ends on its own terms: a semicolon if you're keeping both clauses in one sentence, or a period if you're starting fresh.

This matters most when the adverb drifts. At the front of its clause, it needs a semicolon before it and usually a comma after: 'The DNA survived; however, it was degraded.' Slide 'therefore' into the middle of its own clause instead, and the boundary punctuation moves with the clause break, not with the word: 'the samples, therefore, were unusable' needs a comma on each side of 'therefore' precisely because the real sentence break already happened earlier, at the semicolon or period before it. The adverb is just commenting from inside a clause that's already complete.

This topic is the sharper companion to splices and fragments: a comma splice happens when someone treats 'however' like 'but' and drops only a comma between two full sentences. Spotting conjunctive adverbs by testing whether the words around them could stand alone as full sentences is the fastest way to catch that error.