Targeting spells and instnatly giving shroud, what happens?

[Burr]

I cast a Swords to Plowshares targeting my opponent's creature and he instantly plays a Vines of Vastwood to give it shroud.

Okay, now correct me if I'm wrong but when I cast my StP I have to choose a target (i don't choose the target when it resolves), so if my StP is already targeting his creature and he gives he shroud what happens?

Thanks in advance

[MMogg]

It is countered on resolution.

[Malchar]

When a spell tries to resolve, if all of its targets are illegal then the spell is countered. If it still has some legal targets, it will do as much as possible, but it won't affect targets that have become illegal.

[SlopeeJ]

Kinda been said, but the spell checks for legal targets when you cast it and when it resolves. So its targets are good when cast then it checks again and the targets are illegal because of protection.

Now if you try to target something that was pro white the play would get rewound I believe because you can't cast the spell in the first place because there are no legal targets.

There's no e in "cast". - c

[jrsthethird]

As an addendum, consider Lightning Helix and Searing Blaze. Both spells have an additional effect in addition to 3 damage to the creature. If you would give the creature shroud in response, the Helix would be countered, but the Blaze would not. The reason is because the Helix only has one target, and if that target fails to be legal, the entire spell is countered (so you don't gain life). Searing Blaze, on the other hand, would still resolve, since it requires two targets (the creature and the player), but as long as the player is still a legal target, it will resolve and the player will take 3 damage.

[EssKay]

Is it the same for giving something protection in response, with either Mother of Runes or Emerge Unscathed ?

[Valtrix]

Correct. The same rule applies for protection, namely if a target is illegal upon resolution the spell becomes countered. In this case the protection makes the object untargetable by the spell, similar to shroud.

[jrsthethird]

Anyway, is this mnemonic still correct? I remember it from old rulebooks but don't know if anything has changed.

If you want to remember what protection stops, think DEBT:

An object with protection from _____ cannot be:

D - Damaged
E - Enchanted (Auras)
B - Blocked
T - Targeted

...by permanents, spells, or abilities that are _____.

[on1y0ne]

Anyway, is this mnemonic still correct? I remember it from old rulebooks but don't know if anything has changed.

If you want to remember what protection stops, think DEBT:

An object with protection from _____ cannot be:

D - Damaged
E - Enchanted (Auras)
B - Blocked
T - Targeted

...by permanents, spells, or abilities that are _____.

It is still correct, for the most part. The "E" means it cannot be Enchanted nor can it be Equipped by something with said quality. Also, for completeness, the object with protection cannot be Fortified by a Fortification (http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=126211), not that you would.