What did Paul mean when he said he had not known old Adam except by the law?
Paul meant that the law exposed and identified the old Adamic sin nature. Apart from the commandment, he would not have recognized lust or covetousness as sin. The law hung over that nature, convicting him of breaking it. In particular, 'Thou shalt not covet' showed him what old Adam was all about.
Les explains that he uses "old Adam" in place of the word sin. In the text, Paul says he had not known about old Adam except by the law. The law hangs over the old Adamic nature and constantly tries to convict him that he is breaking it and is contrary to it. It was the law that showed Paul what old Adam was all about. Paul's example is covetousness: "I had not known lust, or covetousness, except the law had said, Thou shalt not covet." Les points out that coveting is the one commandment that must be committed first, since a person cannot kill, steal, commit adultery, or destroy someone's character with false gossip unless he covets. Thus, through the Ten Commandments, coveting is the trigger for breaking the law.