"The mirror has fallen because it is heavy."

Translation: Зеркало упало, потому что оно тяжёлое.

[Anetteberger]

Can someone expain the word "оно"?

[Shady_arc]

"It" refers to the mirror. The mirror is зеркало, which is a neuter noun, so the corresponding personal pronoun is оно.

[fdsimms]

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.

[deactivated user]

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.

[Tim_Shakh]

Right, but it is much more common to put the comma before потому.

[SergeiLependin]

no потому, что is more often met nowerdays.
потому что = так как.
I wrote так как and it marked as a mistake(
I am Russian and I can't pass these tests)