Come Again—One Cannot and Does Not Have To Obey the Law?

God gave us His law in order to obey it. But doesn’t God also say that we cannot serve or obey Him? On the other hand, He only promises us the gift of the Holy Spirit when we obey Him. How can we reconcile these apparent contradictions, and what exactly is the Holy Spirit which is supposed to dwell in us, and what does it accomplish in and through us?