Let's say my opponent casts infernal tutor.
In response to my opponent casting infernal tutor, he sacrifices a lion's eye diamond.
In response to that, can I still counter his infernal tutor?
If I can't, is it because playing a card affects the stack twice? Once to state you are casting it, and twice for the effects of the spell to go on the stack?
Certainly. A counter spell says to "counter target spell". As Infernal Tutor has not yet resolved, it is still a spell and is a valid counter target, even though it's not at the top of the stack.
One correction. Lion's eye diamond's ability is a mana ability and does not use the stack. It has a special timing restriction that only allows it to be used when you could cast an instant. So before you'd counter the spell the infernal tutor would be the only spell on the stack (but as gdiffuser stated even if it were not it could still be countered).
Although a mana ability does not use the stack, it still allows both players priority before resolving what's before it right? Otherwise you could use it to force through a spell
Additionally, note that players always receive priority between each object on the stack resolving. If your opponent had cast One with Nothing in response to Infernal tutor instead of cracking a LED, you would still be able to respond to the tutor both before and after One with Nothing resolved. There's no forcing through a spell just by adding more things to the stack because the stack doesn't resolve all at once.
One correction. Lion's eye diamond's ability is a mana ability and does not use the stack. It has a special timing restriction that only allows it to be used when you could cast an instant. So before you'd counter the spell the infernal tutor would be the only spell on the stack (but as gdiffuser stated even if it were not it could still be countered).
That is incorrect. Priority dictates otherwise. A player may only cast spells when they have priority. If it's your turn and you're not trying to end a phase or step and the stack is empty, you have priority. While you have priority, you can cast as many spells as you want. In order for a spell to resolve, every player must pass priority. So your opponent would cast Infernal tutor, then, because he still has priority, he activates Lion's Eye Diamond. Presumably at that point he passes priority to you, at which point you cast a counter spell. You then pass priority (presumably), and at that point he has the option to cast spells, or pass priority. Assuming he doesn't respond, your Counterspell will resolve, countering Infernal Tutor. Then Lion's Eye Diamond's triggered ability will resolve, causing him to discard his hand and add mana to his pool (assuming he doesn't respond).
Then Lion's Eye Diamond's triggered ability will resolve, causing him to discard his hand and add mana to his pool (assuming he doesn't respond).
Lion's Eye Diamond does not have a triggered ability. It has an activated mana ability, which you may activate that ability whenever you could cast an instant. Since that ability is a mana ability, it resolves immediately after it is activated. Segoth's explanation is correct.
Quote from CR »
602.1. Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation instructions (if any).]"
Quote from CR »
605.1a An activated ability is a mana ability if it meets three criteria: it doesn't have a target, it could put mana into a player's mana pool when it resolves, and it's not a loyalty ability.
Quote from CR »
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.
Let's say my opponent casts infernal tutor.
In response to my opponent casting infernal tutor, he sacrifices a lion's eye diamond.
In response to that, can I still counter his infernal tutor?
If I can't, is it because playing a card affects the stack twice? Once to state you are casting it, and twice for the effects of the spell to go on the stack?
That is incorrect. Priority dictates otherwise. A player may only cast spells when they have priority. If it's your turn and you're not trying to end a phase or step and the stack is empty, you have priority. While you have priority, you can cast as many spells as you want. In order for a spell to resolve, every player must pass priority. So your opponent would cast Infernal tutor, then, because he still has priority, he activates Lion's Eye Diamond. Presumably at that point he passes priority to you, at which point you cast a counter spell. You then pass priority (presumably), and at that point he has the option to cast spells, or pass priority. Assuming he doesn't respond, your Counterspell will resolve, countering Infernal Tutor. Then Lion's Eye Diamond's triggered ability will resolve, causing him to discard his hand and add mana to his pool (assuming he doesn't respond).
Lion's Eye Diamond does not have a triggered ability. It has an activated mana ability, which you may activate that ability whenever you could cast an instant. Since that ability is a mana ability, it resolves immediately after it is activated. Segoth's explanation is correct.