"The mirror has fallen because it is heavy."
Translation:Зеркало упало, потому что оно тяжёлое.
Does it matter if the comma goes before or after потому? I feel like I've seen it both ways but I've never had it explained to me.
I don’t think there's any difference. I believe the comma just indicates intonation (if you pronounce «потому́» emphasised, then put a comma after it; if you don’t, then put it before it), and the meaning is the same.