Jamieson, Fausset and Brown on 1 Timothy 1:8

8. But--"Now we know" (Ro 3:19; 7:14).

law is good--in full agreement with God's holiness and 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 could be attained by the Gospel alone (1Ti 4:8; Tit 1:14), which was the perverted use to which the false teachers put it, but as a means of awakening the sense of sin in the ungodly (1Ti 1:9, 10; compare Ro 7:7-12; Ga 3:21).