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Defending has changed significantly in FIFA 12, and you are penalized much more for being overly-aggressive with your tackles. Use the new defensive situation to your advantage by getting in close and contesting the ball instead of going for the tackle. At the same time, you don't want to be too patient. Not putting enough pressure your opponent will allow them to move down the field easier, putting you on your heels. If you can spare a player without leaving another opponent too open, call in a second defender to help put the pressure on. You can use this to shut down pass lanes or force the defender to lose possession of the ball. Be careful when doing this, as it often leaves an opponent open for a pass. Make sure you are aware of where the opposing players are in the field and shut down any passing lanes. One of the most useful defensive formations in the game is 5-3-2, as it provides good coverage for your defenders. Another solid choice is the 5-2-2-1 formation as it packs the midfield with defenders. In FIFA 12, the key to successfully defending is by anticipating where the opponent is going to pass, and then shutting that pass down. This will be difficult at first, but after some time you'll start to be able to tell which direction a player is going to to based on their direction and speed.
Try not to overreact. Call in a second defender. Use formations to your advantage. Anticipate passes.