To say that the Law has a Pedagogical use is to say that it teaches us our sin. Paul says,
"...I would not have known what it is to covet if the law had not said, 'You shall not covet.'" (Rom 7:7)This is the first function of the Law. To show us our helplessness in keeping it and revealing our sin. Indeed the Law not only reveals sin, but increases our sin.
Then there is the civil use of the Law. This civil use is the way we normally think of laws in general. They restrain evil. Even so, the Law that God gave through Moses in the Mosaic Covenant has this use. It keeps people from sinning.
Why then the law? It was added because of transgressions... (Gal 3:19)Not because people don't want to sin, but because they fear the punishment the Law demands for breaking the Law. However, when opportunity presents itself and the chances of getting away with our lawbreaking is greater then our chances of being caught and punished, we break the Law. So sin is still present in us, but the outworking of that sin is restrained by Law.
<>That's why capital punishment is the only way to keep would-be murderers from becoming murderers and would-be sexual offenders from becoming sexual offenders.< /unabashed conservative politics>
Calvin's third use of the Law raises concern in my mind. The didactic use, or the teaching use, of the Law says that the Law should be used to urge believers into good works. But this is a concept that is never alluded to in the Scriptures. There is no place where believers in the New Covenant are commanded to use the Mosaic Law as a means of provoking good works. Do you want to know why? The Law can't provoke me to good works. It can restrain me from acting on my sinful passions, but it cannot excite the will to do good in me. That is the function of Grace. And that is what Paul tells us time and again in his various treatments of the relationship between Law and Gospel.
But hey, .667 isn't a bad batting average for Calvin, is it?