Do I use a comma before "then" in this example?
Joe said, "If she plans on going, I won't be there, then."
Do I use a comma before "then" in this example?
Joe said, "If she plans on going, I won't be there, then."
She is going to that party? I won't be there, then.
This means I will not be at the party, because she's going (and I have no wish to meet her).
This then is part of an if, then construction, where the if is suppressed: if she's going, then I am not going.
She is going to the party? I won't be there then.
This means that I will not be present at that time.
This then is simply temporal, and the sentence is similar to I won't be there tonight.
By adding a comma, the "then" relates to the condition in the first part of the sentence.
On the other hand - if there's no comma, then this whole sentence doesn't make much sense because the "then" would relate to an exact point in time when Joe would not be there.
The comma version suits better.