But we know that the law is good, if a man use it lawfully;
But - "Now we know" (Romans 3:19; Romans 7:14): as an admitted principle.
Law is good - accordant with God's moral goodness.
If a man - primarily, a teacher; then, every Christian.
Use it lawfully - in its lawful place in the Gospel economy; namely, not as a means of a "righteous man" attaining higher perfection than by the Gospel alone (Titus 1:14) (the perverted use to which the false teachers put it, appending fabulous interpretations of it to the Gospel), but to awaken the sense of sin in the ungodly (1 Timothy 1:9-10: cf. Romans 7:7-12; Galatians 3:21).