The Special Case of Conjunctive Adverbs
Rule 5: A semicolon and a comma are used together when a conjunctive adverb separates two main clauses.
I wanted to go; however, I was too busy.Here is a list of common conjunctive adverbs.
accordingly, furthermore, moreover, similarly, also, hence, namely, still, anyway, however, nevertheless, then, besides, incidentally, next, thereafter, certainly, indeed, nonetheless, therefore, consequently, instead, now, thus, finally, likewise, otherwise, undoubtedly, further, meanwhile.
The check was for more than the balance; consequently, it bounced.
These things really happened; otherwise, I wouldn't have claimed to have seen them.
In these sentences, the conjunctive adverb may look like a coordinate conjunction (and, or, so, but, for), but it is not as strong as a coordinate conjunction. Therefore, the semicolon is used to link the two main clauses, and the comma is used to set off the conjunctive adverb (really an one-word adverb cluster) from its main clause.