Unless a spell or ability is instructing a player to take an action, which player can take actions at any given time is determined by a system of priority. The player with priority may cast spells, activate abilities, and take special actions.
September 30, 2011 · Innistrad
A cost is an action or payment necessary to take another action or to stop another action from taking place. To pay a cost, a player carries out the instructions specified by the spell, ability, or effect that contains that cost.
June 17, 2011 · Commander 2011
A cost is an action or payment necessary to take another action or to stop another action from
February 4, 2011 · Mirrodin Besieged
A cost is an action or payment necessary to take another action or to stop another action from taking place. To pay a cost, a player carries out the instructions specified by the spell, ability, or effect that contains that cost.
October 1, 2010 · Scars of Mirrodin
A cost is an action or payment necessary to take another action or to stop another action from
July 16, 2010 · Magic 2011
A cost is an action or payment necessary to take another action or to stop another action from taking place. To pay a cost, a player carries out the instructions specified by the spell, ability, or effect that contains that cost.
February 1, 2010 · Worldwake
Each player begins the game with a life total of 20.