Here's why:
* "Then" is usually a conjunctive adverb connecting two independent clauses. In this case, the comma goes *before* "then," not after.
* Example: "The dog barked loudly. Then, the cat ran away."
* "Then" can also introduce a dependent clause, but even in this case, a comma is not needed after it.
* Example: "Then, when the sun set, the birds flew home."
However, there are some exceptions:
* If "then" is followed by a parenthetical phrase, you may put a comma after it.
* Example: "Then, in my opinion, the plan won't work."
* If "then" is used for emphasis, a comma after it can be used.
* Example: "Then, and only then, will we know the truth."
In general, it's best to avoid a comma after "then" at the beginning of a sentence unless there's a clear grammatical reason to do so.