Dualcaster Mage — ruling 1

Dualcaster Mage's triggered ability can target (and copy) any instant or sorcery spell, not just one with targets. It doesn't matter who controls it.

Dualcaster Mage — ruling 2

As Dualcaster Mage's triggered ability resolves, it creates a copy of a spell on the stack. The copy isn't cast, so abilities that trigger when a player casts a spell won't trigger.

Dualcaster Mage — ruling 3

The copy will have the same targets as the spell it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal).

Dualcaster Mage — ruling 4

You can't choose to pay any additional costs for the copy. However, effects based on any additional or alternative costs paid for the targeted spell are copied as though those same costs were paid for the copy.

Dualcaster Mage — ruling 5

If the spell is modal (that is, it says "Choose one —" or the like), the copy will have the same mode. You can't choose a different one.

Dualcaster Mage — ruling 6

If the spell has an X whose value was determined as it was cast (like Earthquake does), the copy has the same value of X.

Dualcaster Mage — ruling 7

If the copy says that it affects "you," it affects the controller of the copy, not the controller of the original spell. Similarly, if the copy says that it affects an "opponent," it affects an opponent of the copy's controller, not an opponent of the original spell's controller.

Dualcaster Mage : règles, fonctionnement et interactions (MTG)

Dualcaster Mage is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flash When this creature enters, copy target instant or sorcery spell. You may choose new targets for the copy.

How it works : Dualcaster Mage

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2016· Wizards of the Coast
Dualcaster Mage's triggered ability can target (and copy) any instant or sorcery spell, not just one with targets. It doesn't matter who controls it.
No other card is named in this ruling.
#2June 8, 2016· Wizards of the Coast
As Dualcaster Mage's triggered ability resolves, it creates a copy of a spell on the stack. The copy isn't cast, so abilities that trigger when a player casts a spell won't trigger.
No other card is named in this ruling.
#3June 8, 2016· Wizards of the Coast
The copy will have the same targets as the spell it's copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal).
No other card is named in this ruling.
#4June 8, 2016· Wizards of the Coast
You can't choose to pay any additional costs for the copy. However, effects based on any additional or alternative costs paid for the targeted spell are copied as though those same costs were paid for the copy.
No other card is named in this ruling.
#5June 8, 2016· Wizards of the Coast
If the spell is modal (that is, it says "Choose one —" or the like), the copy will have the same mode. You can't choose a different one.
No other card is named in this ruling.
#6June 8, 2016· Wizards of the Coast
If the spell has an X whose value was determined as it was cast (like Earthquake does), the copy has the same value of X.
No other card is named in this ruling.
#7June 8, 2016· Wizards of the Coast
If the copy says that it affects "you," it affects the controller of the copy, not the controller of the original spell. Similarly, if the copy says that it affects an "opponent," it affects an opponent of the copy's controller, not an opponent of the original spell's controller.
No other card is named in this ruling.