Did the Law have power to help mankind keep it?
No. The Mosaic Law had no power to enable mankind to keep it; it could not prevent stealing or adultery, and all it could do was condemn. It was perfect from God's perspective but faulty from mankind's because man could not keep it, necessitating a new covenant.
Les teaches that Hebrews 8:7-8 introduces the first covenant, the covenant of Law, the Law of Moses. The word "if" in verse 7 implies that it was faulty, not faultless; if it had been faultless, there would be no place sought for a second. It was perfect from God's perspective but not from mankind's. Finding fault with them, God promised a new covenant with the house of Israel and the house of Judah, not according to the covenant He made with their fathers. From man's perspective, the Mosaic Law was faulty because man could not keep it. Les emphasizes that the Law had no power to help mankind keep the Law; it could not keep a man from stealing or committing adultery; all it could do was condemn him. By contrast, Romans 8 speaks of living under the Law of the Spirit, and the Spirit indwells believers, as seen in I Corinthians 3 and 6.