Turning a mana

[Gabriel Gusso]

My opponent is going to cast a creature with cost 4B and he owns 4B lands. When he declares what he is going to cast, may i turn one of his manas with an instant in order to prevent him from casting it?

[Mlerner12]

If he has already tapped the land and said "Casting blah blah blah," I believe you cannot. However, when he starts tapping the mana, you can, as long as the mana has not already been tapped. That is, of course, up for disproval, if that is wrong.

[Keyeto]

As long as you're doing so before they tap their land for mana, yes.

And I'm hoping you mean the cost is {B}{B}{B}{B}, and not {4}{B}, correct?

[Double-O-Scotch]

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.

[Gabriel Gusso]

Yup, {B}{B}{B}{B}. So I cannot anull a magic by turning his controllers mana right?

[Anoobass]

Couldn't he tap in response to you targeting it anyway, so it's useless?

[Double-O-Scotch]

If you did so on your opponents upkeep, or in response to your opponent doing something (basically whenever you, as the non active player, next receive priority) yes. You could change a land. However, if your opponent simply adds {B}{B}{B}{B} and casts a creature spell with that mana, changing a land type or {twiddle}ing a land is useless as it's already too late.

[Double-O-Scotch]

"Mana abilities can't be targeted"

[Mlerner12]

"Mana abilities can't be targeted"

No the can't, but if the land is untapped they can tap the land so it can't be tapped. That targets the land.

[Double-O-Scotch]

Yes, but the land being targeted can be tapped for mana in response.

[Double-O-Scotch]

And you can't respond to tapping a land for mana.

[Double-O-Scotch]

You can also draw a card on your draw phase. What's your point? Lol

[Pleeb]

If you want to tap an opponent's land to prevent them from casting spells, you have to do it before a change of phases where the mana pool empties. For main phase this means taping their land at the end of the draw phase. For the combat phase this means taping it during the first main phase etc.  the only other option I can think of is to wait until priority is passed to you, tap his lands and then cast a {power sink} or some other spell that forces him to use the mana (won't work if he's casting instants).