The verse records as such:
Exodus 18:16 (ESV) when they have a dispute, they come to me and I decide between one person and another, and I make them know the statutes of God and his laws.”
What I'm confused about is that God does not make known the law until ch. 20 so what law does Moses understand and is making known to the people?
A small observation; Genesis 18:19 says
"For I have chosen him, so that he may command his children and his household after him to keep the way of the LORD by doing righteousness and justice, so that the LORD may bring upon Abraham what He has spoken about him."
This was the way for which they would have been destroyed in not keeping while in Egypt before even coming out.
Ezekiel 20:6-9
6 In the day that I lifted up mine hand unto them, to bring them forth of the land of Egypt into a land that I had espied for them, flowing with milk and honey, which is the glory of all lands:
7 Then said I unto them, Cast ye away every man the abominations of his eyes, and defile not yourselves with the idols of Egypt: I am the LORD your God.
8 But they rebelled against me, and would not hearken unto me: they did not every man cast away the abominations of their eyes, neither did they forsake the idols of Egypt: then I said, I will pour out my fury upon them, to accomplish my anger against them in the midst of the land of Egypt.
9 But I wrought for my name's sake, that it should not be polluted before the heathen, among whom they were, in whose sight I made myself known unto them, in bringing them forth out of the land of Egypt.
So Israel had a 'way of the LORD' back then all the while in Egypt that they had to go by, plus a few more a little after that e.g in Exod 13:3 about the first borns, the unleaven bread, the Sabbath and general details about their feast days
Exod 13:3
5 And when the Lord brings you into the land of the Canaanites, the Hittites, the Amorites, the Hivites, and the Jebusites, which he swore to your fathers to give you, a land flowing with milk and honey, you shall keep this service in this month.
6 Seven days you shall eat unleavened bread, and on the seventh day there shall be a feast to the Lord.
7 Unleavened bread shall be eaten for seven days; no leavened bread shall be seen with you, and no leaven shall be seen with you in all your territory.
This is what Moses apparently 'overstates' as the laws and the statutes unaware that the biggest part was yet on the way.
The ESV translated the Hebrew word torah in the Masoretic Text as "law", which certainly refers to the Law of Moses, but it can also simply refer to a "teaching". Similarly, the underlying Hebrew for "statute" can mean any sort of communicated instruction of what one should do.
I think the previous verse must be included to make understand 18:16 -
Exodus 18:15–16 (JPS Tanakh)
Moses replied to his father-in-law, “It is because the people come to me to inquire of God. When they have a dispute, it comes before me, and I decide between one person and another, and I make known the laws and teachings of God.”
Thus, Moses is referring to what he is instructed by God at the time, or what may have been instructed by God on prior occasions that could be relevant.
There are other occasions where Moses and others were guided by God directly and immediately rather than through the written Law.
Leviticus 24:10–13 (JPS Tanakh)
There came out among the Israelites one whose mother was Israelite and whose father was Egyptian. And a fight broke out in the camp between that half-Israelite and a certain Israelite. The son of the Israelite woman pronounced the Name in blasphemy, and he was brought to Moses—now his mother’s name was Shelomith daughter of Dibri of the tribe of Dan—and he was placed in custody, until the decision of the LORD should be made clear to them.
And the LORD spoke to Moses, saying ...
Numbers 9:6–9 (Tanakh)
But there were some men who were unclean by reason of a corpse and could not offer the passover sacrifice on that day. Appearing that same day before Moses and Aaron, those men said to them, “Unclean though we are by reason of a corpse, why must we be debarred from presenting the LORD’s offering at its set time with the rest of the Israelites?” Moses said to them, “Stand by, and let me hear what instructions the LORD gives about you.”
And the LORD spoke to Moses, saying ...
Numbers 15:32–35 (Tanakh)
Once, when the Israelites were in the wilderness, they came upon a man gathering wood on the sabbath day. Those who found him as he was gathering wood brought him before Moses, Aaron, and the whole community. He was placed in custody, for it had not been specified what should be done to him. Then the LORD said to Moses ...
