If I flash in Teferi, Time Raveler, can its passive ability counter a spell on the stack?

Question

If I play Teferi, Time Raveler at instant speed, by way of, for example, Raff Capashen, Ship's Mage, will Teferi's passive ability counter a spell being played by an opponent at a time when they can not cast sorceries?

The ability in question reads

Each opponent can cast spells only any time they could cast a sorcery.

So if, for example, my opponent casts Vraska's Contempt on Raff during my end phase, and I respond by flashing in Teferi, does the Vraska's Contempt resolve?

My suspicion is that the opponent's spell will, in fact, resolve, because it has already been cast (i.e. is already on the stack) when Teferi resolves and hits the board. Is this correct?

Answer 1

Your suspicion is correct, Teferi prevents opponent's cards from being cast outside the main phases, but if the opponent's spell is already on the stack, then it has already been cast, and as such Teferi's passive will not have any effect on it. They could not, however, respond to the stack again after Teferi resolves.

Answer 2

After casting Teferi, Time Raveler and it resolves if you were to then cast negate on your opponent's Vraska's contempt your opponent would not be able to counter your counter spell because of the passive ability, however, the situation you describe would not counter the spell.

To counter a spell you can do so by 2 means countering it, making its targets illegal.

What Teferi does is a restriction, that takes effect "on casting". Same applies to Chalice of the Void. A spell that is on the stack has been cast already, the stack is a zone, so you can move spells from the stack with the use of other spells. You can for example use Glorious End to move it to exile without resolving it, or any other such ability.

But you wanted to do can't be done as there is nothing on Teferi's ability that counters a spell, it only restricts the casting of the spell.