If I have two mana untaped (enough for one negate) and my opponent casts Volcanic Offering and destroys one of my untaped lands, I assume I can't use Negate to counter it because of stack order correct?
If Volcanic Offering has resolved, there's no chance to counter it anyway, but the lands are destroyed as part of resolution of the spell. The lands are still around whilst the Offering is on the stack and it can be Negated even if it targets one of your only two untapped nonbasic lands.
Your opponent has cast a spell which has some targets. One of which is your non-basic land. This spell is now on the stack. The top most item of the stack cannot resolve until each player passes priority. So first the active player gets priority. In general, this is whomever's turn it is. So if it's your turn you get priority. Your opponent's turn would mean they get priority. Whomever gets priority gets first opportunity to act. In your case, you would want to act by casting negate. Your opponent would want to do nothing and pass priority. If you decide to cast negate, this process of priority happens all over again, and negate won't resolve (and counter the Volcanic Offering) until both players have passed priority.
Remember though, spells are countered in their entirety. You can't counter just damage part or just the land destruction part, you have to counter the whole spell.
ALSO! If you wanted to save your negate you could tap your lands in response and give yourself two mana. This is known as "floating mana" and you will still have access to the mana until your manapool empties. This means if your opponent was trying to blow up your land so you counter their next spell, you still could.
When you counter a spell, you do it before it starts resolving (trying to counter it afterwards isn't actually possible within the rules of the game). In your example Volcanic Offering hasn't done anything yet at the time you're allowed to respond with Negate.
I̟̥͍̠ͅn̩͉̣͍̬͚ͅ ̬̬͖t̯̹̞̺͖͓̯̤h̘͍̬e͙̯͈̖̼̮ ̭̬f̺̲̲̪i͙͉̟̩̰r̪̝͚͈̝̥͍̝̲s̼̻͇̘̳͔ͅt̲̺̳̗̜̪̙ ̳̺̥̻͚̗ͅm̜̜̟̰͈͓͎͇o̝̖̮̝͇m̯̻̞̼̫̗͓̤e̩̯̬̮̩n͎̱̪̲̹͖t͇̖s̰̮ͅ,̤̲͙̻̭̻̯̹̰ ̖t̫̙̺̯͖͚̯ͅh͙̯̦̳̗̰̟e͖̪͉̼̯ ̪͕g̞̣͔a̗̦t̬̬͓͙̫̖̭̻e̩̻̯ ̜̖̦̖̤̭͙̬t̞̹̥̪͎͉ͅo͕͚͍͇̲͇͓̺ ̭̬͙͈̣̻t͈͍͙͓̫̖͙̩h̪̬̖̙e̗͈ ̗̬̟̞̺̤͉̯ͅa̦̯͚̙̜̮f͉͙̲̣̞̼t̪̤̞̣͚e̲͉̳̥r͇̪̙͚͓l̥̞̞͎̹̯̹ͅi͓̬f̮̥̬̞͈ͅe͎ ̟̩̤̳̠̯̩̯o̮̘̲p̟͚̣̞͉͓e͍̩̣n͔̼͕͚̜e̬̱d̼̘͎̖̹͍̮̠,͖̺̭̱̮ ̣̲͖̬̪̭̥a̪͚n̟̲̝̤̤̞̗d̘̱̗͇̮͕̳͕͔ ͖̞͉͎t̹̙͎h̰̱͉̗e̪̞̱̝̹̩ͅ ̠̱̩̭̦p̯̙e͓o̳͚̰̯̺̱̰͔̘p̬͎̱̣̼̩͇l̗̟̖͚̠e̱͉͔̱̦̬̟̙ ̖͚̪͔̼̦w̺̖̤̱e͖̗̻̦͓̖̘̜r̭̥e͔̹̫̱͕̦̰͕ ̗͔̠p̠̗͍͍̱̳̠r̰͔͎̰o͉̥͓̰͚̥s̟͚̹̱͔̣t͉̙̳̖͖̪̮r̥̘̥͙̹a͉̟̫̟̳̠̟̭t͈̜̰͈͎e̞̣̭̲̬ ͚̗̯̟͙i͍͖̰̘̦͖͉ṇ̮̻̯̦̲̩͍ ̦̮͚̫̤t͉͖̫͕ͅͅh͙̮̻̘̣̮̼e͕̺ ͙l͕̠͎̰̥i̲͓͉̲g̫̳̟͈͇̖h̠̦̖t͓̯͎̗ ̳̪̘̟̙̩̦o̫̲f̙͔̰̙̠ ̹̪̗͇̯t͖̼̼͉͖̬h̹͇̩e͚̖̺̤͉̹͕̪ ͚͓̭̝̺G͎̗̯̩o̫̯̮̟̮̳̘d̜̲͙̠-̩̳̯̲̗̜P̹̘̥͉̝h͍͈̗̖̝ͅa͍̗̮̼̗r̜̖͇̙̺a̭̺͔̞̳͈o̪̣͓̯̬͙̯̰̗h̖̦͈̥̯͔.͇̣̙̝
Your opponent has cast a spell which has some targets. One of which is your non-basic land. This spell is now on the stack. The top most item of the stack cannot resolve until each player passes priority. So first the active player gets priority. In general, this is whomever's turn it is. So if it's your turn you get priority. Your opponent's turn would mean they get priority. Whomever gets priority gets first opportunity to act. In your case, you would want to act by casting negate. Your opponent would want to do nothing and pass priority. If you decide to cast negate, this process of priority happens all over again, and negate won't resolve (and counter the Volcanic Offering) until both players have passed priority.
Remember though, spells are countered in their entirety. You can't counter just damage part or just the land destruction part, you have to counter the whole spell.
ALSO! If you wanted to save your negate you could tap your lands in response and give yourself two mana. This is known as "floating mana" and you will still have access to the mana until your manapool empties. This means if your opponent was trying to blow up your land so you counter their next spell, you still could.
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