A sign is hanging up at work: Empty the trash, then line the basket with a new bag.
I saw the forecast for my local area: Rain will end this morning, then it will be partly sunny this afternoon.
Both of these sentences show the mis-use of punctuation for conjunctive adverbs.
Conjunctive adverbs, also called adverbial conjunctions, join independent clauses (clauses that can stand alone as full sentences) and should NOT be joined by a comma. This mistake is one that is made much too often, especially with the adverbial conjunction then. By contrast, coordinating conjunctions (and, but, so, for, yet, or, nor) joining two independent clauses DO use a comma between them.
The correct punctuation when using a conjunctive adverb is a semicolon before the conjunctive adverb and a comma after it.
Rain will end this morning; then, it will be partly sunny this afternoon.
Empty the trash; then, line the basket with a new bag.
The same punctuation rules apply for all other conjunctive adverbs. Some of the most common of them are otherwise, therefore, in fact, thus, hence, and however.