Commas are written ways to (attempt to) record the intonation contour of a sentence; there are many varieties of such contours, with different pragmatic and semantic effects. The comma is a rather primitive way of doing it -- one binary on/off mark to represent all continuous variations in tone, volume, and rhythm. The basic comma rule is: if you hear it, you write it. If you don't hear it, you don't write it.
But what are you sposta listen for? Basically, it's a sine wave. From the normal indicative
mid tone to the
high tone, then down to
low and back to
mid again, very fast, usually no more than two syllables. The comma is placed where the intonation happens. There is no pause in the sound stream, but it's often called a "pause" because it functions to separate constituents of the utterance.
The prototype intonation curve is the rhythm and melody of a list, as in counting:
- ... fifty-one, fifty-two, fifty-three, fifty-four, fifty-five, ...
In the presenting sentence
- I haven't played much Risk, but if we do, you can imagine what would happen.
the first clause is a complete sentence and ends with full stop intonation. It should end with a semicolon instead of a comma in print; comma intonation is wrong here; it's a classic comma splice.
- I haven't played much Risk; but if we do, you can imagine what would happen.
And now why do we need a but? Simply putting these two clauses together without a conjunction tells the story better; but indicates surprise, but there is no surprise in this conclusion. And, with no but, the issue of over-commatizing the clause is averted.
- I haven't played much Risk; if we do, you can imagine what would happen.
After contrastive do, there's another issue. Some proform is needed beside do, since the subject has changed from I in the first clause to we in the next, so the VP deletion is not quite complete.
Better would be something like
- I haven't played much Risk; if we do play it, you can imagine what would happen.
Finally, will (contracted, naturally, to what'll) is better than would here; there's no need to invoke irrealis conditions repeatedly in a hypothetical clause -- it's already imaginary and can't get any more so.
- I haven't played much Risk; if we do play it, you can imagine what'll happen.