If the spell that’s copied is modal (that is, it says “Choose one —” or the like), the copy will have the same mode. Its controller can’t choose a different one.

The copy that Bonus Round’s ability creates is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell (like Bonus Round’s ability itself) won’t trigger.

The controller of the spell that caused Bonus Round’s ability to trigger also controls the copy.

The controller of the copy can’t choose to pay any additional costs for the copy. However, effects based on any additional costs that were paid for the original spell are copied as though those same costs were paid for the copy too.

Bonus Round’s ability will copy any instant or sorcery spell, not just one with targets.

If the spell that’s copied has an X whose value was determined as it was cast, the copy has the same value of X.

A copy is created even if the spell that caused Bonus Round’s ability to trigger has been countered by the time that ability resolves. The copy resolves before the original spell.

The copy will have the same targets as the spell it’s copying unless its controller chooses new ones. That player may change any number of the targets, including all of them or none of them. The new targets must be legal.