I was writing a compound sentence, that required a 'Then' after the coordinating conjunction. I was wondering if the 'Then' needed a comma before and after it, such as thus:
"It is best to review once or twice, and, then, move on."
If it should be included or omitted, I'd appreciate any tips.
I would suggest the parenthetical commas you propose actually change the meaning of then. This is because they change the stress afforded the word and, to a lesser extent, its pronunciation.
Without them, "review it and then move on", then means "after that" [Oxford sense 2]. In this case, then receives the stress rather than and.
With the commas, "review it and, then, move on", then means "in that case; therefore" [Oxford sense 3]. In this sentence, because then is effectively in parentheses, and receives the stress and then is relegated to secondary importance. It is this which changes the meaning.
In passing, if the then is required, it cannot be parenthetical, cannot be contained in commas, and cannot have the meaning of "therefore". However, it's not actually required even when it means "after that", because the two actions have to be carried out in sequence: "review it and move on". Thus it is when it does have the meaning of "therefore" that it is required, and in that case it needs the commas to give it that meaning. It's rather paradoxical that a required word must apparently be parenthetical to give it its required meaning.
