I have a Creature spell and Duress in my hand, I cast the creature spell. My opponent starts tapping lands to cast a counter spell on the stack. Can I interrupt him and cast Duress to make him discard the instant card from his hand?
To answer the titular question:
No, you can not play an instant when your opponent taps mana to cast an instant.
This when a player is allowed to preform a certain action is a fairly in depth topic, and if you want to learn more about it I would recommend the following sources:
- In plain English, how does casting spells and using creature abilities work with the stack and priority?
- Magic's Comprehensive Rules
However to summarize, you can not cast an instant "in response" to your opponent tapping their lands because of two reasons:
- You do not receive priority (and therefore can not act) until after your opponent finishes casting a spell. This is outlined throughout 601.2 (Rules for casting a spell) with the relevant part here (Italicized text mine):
601.2i: Once the steps described in 601.2a–h (Steps to casting a spell including, paying costs and choosing targets) are completed, effects that modify the characteristics of the spell as it’s cast are applied, then the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell’s controller had priority before casting it, he or she gets priority.
- Mana abilities (such as tapping lands) do not use the stack and can not be responded to. This is outlined here:
605.3b: An activated mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after it is activated. (See rule 405.6c.)
To address the specific example in the body of your question, No you can not cast Duress to force your opponent to discard the counter spell before he finishes casting it. In addition to the reasons above, you can not cast Duress in response to anything as it is a sorcery. Sorceries may only be cast during one of your main phases when the stack is empty (Comprehensive Rules 307).
No, you cannot cast Duress in this situation to remove the counter spell.
First, Duress is a sorcery, not an instant.
Secondly, once an opponent begins to cast a spell, you can't stop that spell until it is out of their hand and on the stack.
Third, even if you somehow could cast Duress as an instant, you cast your creature and say as you're casting it "... and I'll hold priority", and cast Duress, they could still cast their counterspell, since your Duress has to resolve before it can remove the card from their hand.
No, because tapping a land for mana is a "mana ability", which does not use the stack and cannot be responded to.
605.1. Some activated abilities and some triggered abilities are mana abilities, which are subject to special rules. ...
605.1a An activated ability is a mana ability if it meets all of the following criteria: it doesn’t have a target, it could add mana to a player’s mana pool when it resolves, and it’s not a loyalty ability. (See rule 606, “Loyalty Abilities.”)
605.3b An activated mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after it is activated. (See rule 405.6c.)
Almost all sources of mana are mana abilities and as such can not be responded to.
A small number of abilities that generate mana are not "mana abilities" for various reasons, 'though. For example Deathrite Shaman's first ability is not a mana ability, because it requires a target.
That means if you opponent used that ability to get some mana, you could respond to that activation with an instant of your own (not with Duress, though, since that's a Sorcery).