Q. I am in a writing/editing group, and everyone here uses then as a conjunction. For example: “I plan to work from home until he is finished then I will come to the office.” Sometimes they put a comma before then. Will you please explain how then is to be punctuated? They are editing their customers’ documents so that they now reflect incorrect usage.

A. Although then is not a conjunction in your sentence, it looks like one because the true conjunction (and or but) is omitted but implied: I plan to work from home [and] then I will come to the office. The comma is necessary because it indicates the implied conjunction and prevents a run-on sentence; a semicolon would be even better.