What does it mean to forgive someone? And what about anger? Tim Keller explores the meaning of  being angry and not sinning, while discussing the nature of forgiveness.

Therefore, putting away falsehood, let every one speak the truth with his neighbor, for we are members one of another.  Be angry but do not sin; do not let the sun go down on your anger, and give no opportunity to the devil.

Let the thief no longer steal, but rather let him labor, doing honest work with his hands, so that he may be able to give to those in need.  Let no evil talk come out of your mouths, but only such as is good for edifying, as fits the occasion, that it may impart grace to those who hear.  And do not grieve the Holy Spirit of God, in whom you were sealed for the day of redemption.  Let all bitterness and wrath and anger and clamor and slander be put away from you, with all malice,  and be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you. Ephesians 4

Having an attitude of compassionate forgiveness is more than just an external standard we are meant to live up to, but is the organic fruit of a heart that has been melted by receiving the forgiveness of Christ. ( from the website--http://sermons2.redeemer.com/sermons/forgiving-and-forgiven)

The link to the audio file of  the sermon  is below-- http://sermons2.redeemer.com/sites/sermons2.redeemer.com/files/sermons/Forgiving_and_Forgiven_0.mp3