One explanation—and the best succinct one I know of—comes from Chapter XIX of the Westminster Confession of Faith (WCF). For Reformed Christians in general, this ought to summarize what we believe. For Presbyterian office bearers in particular, this is what you swear to uphold. For Christians at large, there are plenty of Bible references in the WCF so you can see for yourself if these things are so.
I’ll ask the questions, and let Chapter XIX give the answers. Whenever the text is in italics that means I’m quoting directly from the Confession.
*******Me: Hey, thanks for being willing to meet with me WCF. I know you are busy and very old, so I’ll try not to take up too much of your time. I just have a few questions about the law. For starters, where did the law come from? Was it just added after the fall?
WCF: God gave to Adam a law, as a covenant of works, by which he bound him and all his posterity to personal, entire, exact, and perpetual obedience, promised life upon the fulfilling, and threatened death upon the breach of it.
Me: Kind of wordy, WCF, but I think I get it. God gave Adam the law from the very beginning, even before sin entered the world.