Grace means that God does something for me; law means that I do something for God. God has certain holy and righteous demands which He places upon me: that is law.
"The law is holy, and the commandment holy, and righteous, and good" (Rom. 7:12). No, there is nothing wrong with the Law, but there is something decidedly wrong with me. The demands of the Law are righteous, but the person upon whom the demands are made is unrighteous.
We are all sinners by nature. If God asks nothing of us, all seems to go well, but as soon as He demands something of us, the occasion is provided for a grand display of our sinfulness.
He knows that I am weakness incarnate; that I can do nothing. The trouble is that I do not know it. God has to do something to convince us of the fact. Romans 7:7 "I had not known sin, except through the law..."
The law was given in the full knowledge that we would break it; and when we have broken it so completely as to be convinced of our utter need, then the Law has served its purpose. It has been our schoolmaster to bring us to Christ, that in us He may Himself fulfill it (Gal. 3:24).
Praise God, He is the Lawgiver on the Throne, and He is the Lawkeeper in my heart. He who gave the Law, Himself keeps it. He makes the demands, but He also meets them.
Bits and pieces from The Normal Christian Life, chapter 9 by Watchman Nee