What tap effect has precedence?

Question

Suppose I control Wellwisher:

{T}: You gain 1 life for each Elf on the battlefield.

And my friend controls Ballynock Trapper:

{T}: Tap target creature.

Whenever you cast a white spell, you may untap Ballynock Trapper.

My question is who has precedence? Is there any way I can get my effect off and not be interrupted by his effect?

If I also control Seedborn Muse...

Untap all permanents you control during each other player's untap step.

... Would that change who has precedence?

I get the feeling that at least on my turn I would be able to use my abilities before he can use his, am I wrong on this?

Answer 1

The short answer is that no matter what your opponent does with their Trapper, they cannot prevent you from activating your Wellwisher and gaining the life.

The long answer involves explaining some of the key systems in the Magic rules.

Magic resolves spells and abilities using a system called "the stack". When you cast a spell or ability, it goes on top of the stack, and then any player and put an instant or ability on the stack. Then once nobody puts anything else on the stack, the spells and abilities resolve from the top down. This allows player to respond to spells or abilities with other spells or abilities, and the response resolves first. More information can be found on the relevant wiki page.

Magic also has a particular meaning for the word "priority", which is different from how you used it in the question. Basically, only one player gets priority at a time, and each player gets priority before any spell or ability resolves, to give each player the opportunity to respond. You gain priority very frequently, including at the end of almost every step of each player's turn, whenever a spell or ability is put on the stack, and whenever a spell or ability resolves from the stack.

Whatever the situation is, either you have priority, or your opponent does. In the first case, the sequence of events looks like this:

1. You activate your Wellwisher's ability. This consists of the following steps (often condensed into one action):
   • You announce that your are activating the ability.
   • You tap Wellwisher to pay for the cost of the ability.
2. The ability is now activated and on the stack. You pass priority. Your opponent now has the opportunity to respond.
3. Your opponent activates their Trapper's ability. This has a similar sequence of actions:
   • They announce that they are activating the ability.
   • They choose Wellwisher as the target of the ability.
   • They tap their Trapper to pay for the cost of the ability.
4. Now the Wellwisher's ability is on the stack, and the Trapper's ability is on top of it on the stack. Both players pass priority (this is often assumed to be the case when neither player announces any responses).
5. The Trapper's ability resolves, and it taps Wellwisher. Wellwisher is already tapped, so this doesn't do anything.
6. Both players pass priority again, and the Wellwisher's ability resolves. You gain a bunch of life.

In the second case, where your opponent has priority, the sequence of events looks very similar:

1. Your opponent activates his Trapper's ability, targeting Wellwisher and tapping the Trapper to pay the cost.
2. The Trapper's ability is now on the stack and activated. Your opponent passes priority.
3. You activate your Wellwisher's ability, tapping Wellwisher to pay the cost.
4. Now the Trapper's ability is on the stack, and the Wellwisher's ability is on top of it on the stack. Both players pass priority.
5. The Wellwisher's ability resolves, and you gain a bunch of life.
6. Both player's pass priority again, and the Trapper's ability resolves, tapping Wellwisher. Wellwisher is already tapped, so this doesn't do anything.

As you can see, it doesn't matter who activates their ability first. Either way, you can gain the life from Wellwisher.

Answer 2

You would in fact be able to activate Wellwisher's ability successfully on either player's turn. The Wellwisher would be untapped during your untap step; no player obtains priority during the Untap step. The next opportunity to activate abilities is during your Upkeep step. Because the active player has first priority, you can activate the Wellwisher then, paying its cost (tapping Wellwisher); your opponent cannot respond until after this has already occurred, while Wellwisher's ability is on the stack. At this point, Wellwisher's ability (on the stack) is essentially a separate entity, and the Trapper tapping Wellwisher has no effect upon the ability's resolution.

On your opponent's turn, this is no different. Seedborn Muse would untap your Wellwisher as expected; however, your opponent would receive first priority during the Upkeep step, as it is their turn. If they activate Ballynock Trapper's ability targeting your Wellwisher, then both players must gain and pass priority before the ability resolves. When this occurs, you can activate your Wellwisher's ability, which as a cost will tap itself and place the ability on the stack. The Wellwisher's ability will (unless otherwise prevented, by Azorious Guildmage, etc.) resolve, and then the Trapper's ability will resolve, tapping the Wellwisher to no effect.

http://wizards.custhelp.com/app/answers/detail/a_id/500

In either case, at some point, before anything can resolve which would destroy, tap, or otherwise affect your Wellwisher, you can activate the ability; after this, the ability on the stack is effectively independent of Wellwisher, and will not be prevented if Wellwisher is tapped or even destroyed.

There are some exceptions to the previously stated rule -- for example, if it's your opponent's turn a Sudden Death targeting Wellwisher during their Upkeep would prevent Wellwisher from being able to activate its ability while Sudden Death is on the stack, and Wellwisher would be destroyed when Sudden Death resolved. On your turn, however, you could immediately activate Wellwisher's ability (as you have priority); after you pass priority, your opponent could respond with Sudden Death, which would almost certainly successfully kill Wellwisher, but the Wellwisher's ability would still be on the stack and would resolve as expected.

(My apologies for including this as a separate answer; if I could currently comment, my two cents would take the form of constructive criticism addressed at the excellent, but not yet perfected, answer by murgatroid99.)