You may choose not to untap Tawnos's Coffin during your untap step.
, : Exile target creature and all Auras attached to it. Note the number and kind of counters that were on that creature. When Tawnos's Coffin leaves the battlefield or becomes untapped, return that exiled card to the battlefield under its owner's control tapped with the noted number and kind of counters on it. If you do, return the other exiled cards to the battlefield under their owner's control attached to that permanent.
10/4/2004 The creature returns to the battlefield tapped. It does not return to the battlefield and then tap afterwards.
9/16/2007 If the targeted creature is a token, it will cease to exist after being exiled. Any Auras that were attached to it will remain exiled forever.
9/16/2007 If the exiled card is returned to the battlefield and, for some reason, it now can’t be enchanted by an Aura that was also exiled by Tawnos’s Coffin, that Aura will remain exiled.
9/16/2007 If Tawnos’s Coffin leaves the battlefield before its ability has resolved, it will exile the targeted creature forever, since its delayed triggered ability will never trigger.
4/1/2008 Because the new wording doesn’t use phasing, the exiled card will suffer from summoning sickness upon its return to the battlefield.
4/1/2008 The effect doesn’t care what types the card has after it is exiled, only that it have been a creature while on the battlefield.