Here's why:
* "Then" is an adverb that indicates sequence or order. It doesn't usually function as a conjunction that joins two independent clauses.
* Commas separate independent clauses (clauses that could stand alone as sentences). Since "then" typically introduces a dependent clause, a comma isn't needed.
Example:
* Correct: Then she went to the store.
* Incorrect: Then, she went to the store.
Exceptions:
There are a few situations where you might use a comma after "then":
* If "then" introduces a long or complex clause: This can help with clarity.
* If "then" is followed by a contrasting or surprising element: A comma can help emphasize the change in direction.
However, these are rare cases. In most instances, you'll leave out the comma.