If you have Teferi on the battlefield and there's a Split Second spell on the stack (such as Sudden Death), you can't Suspend creatures until after the Split Second spell resolves.
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 the player doesn't have a sorcery they are able to cast, or if a rule or effect allows a sorcery to be cast at another time.
If you have Teferi on the battlefield, you may exile creature cards in your hand with suspend any time you could cast an instant.
If a spell or ability lets an opponent cast a card as part of its effect (such as suspend and rebound do), that opponent can't cast that card since the currently resolving ability is still on the stack. This is true even if that card is an instant.