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

While you control Teferi, you may exile creature cards in your hand with suspend any time you could cast an instant.

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.

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 a spell or ability lets an opponent cast a spell as part of its effect (such as suspend and rebound do), that opponent can’t cast that spell since the resolving ability is still on the stack. This is true even if the spell is an instant.

Playtest cards aren’t legal for play in any tournament format other than Mystery Booster Limited formats. On the other hand, we expect they will spice up a wide variety of non-tournament games (as long as everyone’s on the same page about using them!).

Playtest cards use a modified version of game symbols, such as {T} and {W}. These modified symbols should be treated as the standard symbols during play.

For many playtest cards, you’ll need to make a generous assumption that basic game rules would be updated to allow them to work. The Mystery Booster 2 Playtest Card Notes section (reproduced here in individual Gatherer rulings) provides guidance for fitting these cards into the existing rules structure.