Intéraction Téféri et Dragon hypersonique

101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.

The last ability means that in order for an opponent to cast a spell, it must be that opponent's turn, during a main phase, and the stack must be empty. This is true even if the player doesn't have a sorcery he or she is able to cast, or if a rule or effect allows a spell to be cast at another time

307.5. If a spell, ability, or effect states that a player can do something only “any time he or she could cast a sorcery,” it means only that the player must have priority, it must be during the main phase of his or her turn, and the stack must be empty. The player doesn’t need to have a sorcery he or she could actually cast. Effects that would prevent that player from casting a spell or casting a sorcery don’t affect the player’s capability to perform that action (unless the action is actually casting a spell or casting a sorcery).

9/25/2006: If an ability lets an opponent cast a spell as part of its effect (such as Isochron Scepter's ability does), that opponent can't cast that spell since the currently resolving ability is still on the stack.