Teferi, Mage of Zhalfir

Teferi's 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 a rule or effect allows a sorcery to be cast at another time.

If an effect allows an opponent to cast spells any time they could cast an instant (for example, if your opponent also controls Teferi), the restriction of Teferi's last ability takes precedence over that permission.

If a spell on the stack has split second while you control Teferi, you can't suspend creature cards since split second stops you from casting them, even though suspending them is a special action.