Adam Clarke on 1 Timothy 1:8

But we know that the law is good - The law as given by God, is both good in itself and has a good tendency. This is similar to what the apostle had asserted, Rom 7:12-16 : The law is holy; and the commandment is holy, just, and good; see the note on Rom 7:12, etc.

If a man use it lawfully - That is, interpret it according to its own spirit and design, and use it for the purpose for which God has given it; for the ceremonial law was a schoolmaster to lead us unto Christ, and Christ is the end of that law for justification to every one that believes. Now those who did not use the law in reference to these ends, did not use it lawfully - they did not construe it according to its original design and meaning.