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; therefore, they should NOT be joined by a comma. This mistake is one that is made much too often, especially with the adverbial conjunction then.
The correct punctuation 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.