If the conjunction joins two independent clauses, insert a comma. If the subordinate clause is first, put a comma after it. If the independent clause is first, put no punctuation before the comma. Subordinating conjunctions cannot be used unless they are joined to an independent clause. It's a fragment.
Conjunctive adverbs join and relate independent clauses.
Therefore , I planned to sign up for 20 credit hours. I only signed up for 3 credit hours, however . Put a period or semicolon before the conjunctive adverb when it is at the beginning of a sentence. A comma follows the conjunctive adverb. Never combine to independent clauses with a comma and a conjunctive adverb.