"The helicopter will land in a minute or two ."
"Why can't we see it, then?
Do I need that comma before the then? Why and why not?
I got both versions on Google:
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The comma can represent a syntactic pause and/or a change of intonation that accompanies and corroborates a syntactic pause.
A comma would make clear that "then" as used here is not temporal but conditional; temporal and conditional uses of then have different syntactic pauses and intonational contours.
P.S. Its being at the end of the sentence is not the deciding factor. Consider:
**I'm away Monday, but I'll be in town on Tuesday. Can I see the apartment then? [no pause before "thenn"]
The answer on the rental form is incorrect. I have no pets. I can rent the apartment, then?** [pause and different intonation to mark the meaning "in that case"]
No it doesn't. Commas are often used to represent the natural pauses we make when talking, and the phrase "why can't we see it then?" can be spoken naturally with or without the pause before the word "then".
The only time I can think of where commas are required is when you use nonessential relative clauses. Here's an example from the linked website:
My dog Floyd, who eats too much pizza, has developed pepperoni breath.
With a comma before it "then" becomes synonymous with "in that case". Without the comma, "then" is more likely to be a temporal reference (refer to a particular time).
It's visible? Well, why can't we see it, then?
You're free tomorrow? Well the film is on tomorrow afternoon; shall we see it then?
Also +1 TRomano's syntactic pause.