Claudius wants very badly to pray and to ask for forgiveness for his heinous crime. He murdered his brother, and he knows his "offense is rank," so foul and terrible that the stench of it reaches heaven (3.3.40). Despite his desire to pray and be absolved for his sin, he says,
But, O, what form of prayer
Can serve my turn? "Forgive me my foul murder"?
That cannot be, since I am still possessed
Of those effects for which I did the murder.
My crown, mine own ambition, and my queen.
May one be pardoned and retain th' offense? (3.3.55-60)
Thus, Claudius has revealed his total awareness of his sin as well as his unwillingness to part with the things he gained when he committed this sin—the things, in fact, for which he committed the sin. His ambition drove him to kill his brother so that he could take both the crown and his brother's wife, and now he feels that he cannot sincerely repent because he wants to keep those things that he gained. Claudius knows that he has a "bosom black as death" and a "limed soul; he is trapped. He wants to repent but he cannot (3.3.71, 72). Claudius then tries to pray and finds that he cannot truly do so.