When we speak of a particular law being eternal, we are not saying that it is without beginning and without end. Only Hashem is without beginning – everything else has an origin. And so an eternal law is one that starts at a particular point in time, and then carries on forever. Once it is instituted, it is always required to be obeyed.
Examples of eternal laws can be found in the ten commandments found in Exodus 20:
You shall not murder. You shall not commit adultery.
G-d never approves of murder or adultery. It doesn’t matter the culture, it doesn’t matter the era. It doesn’t matter if it is a Jew or a Gentile, it doesn’t matter if it was 1000 years ago or is 1000 years from now. Such heinous acts are never permissible and always sinful in G-d’s eyes.
So when we hear the rabbis speak about the eternal Law of Moses, we need to realize what they are teaching is that each and every individual law that makes up the whole body of law is eternal in duration. If all of the single laws are not eternal in duration, how then can the entire collection of laws be eternal in duration, since the whole is simply the sum of the parts?