“Moses wrote all the words of Yahweh.” Moses wrote down all the laws and regulations that God had spoken from Exodus 20:2-23:19. He would have included the Ten Commandments because God had not written them on tablets yet. Moses wrote a fairly small scroll, which was called “the Book of the Covenant” (Exod. 24:7), and it contained all the laws and regulations that God had spoken in Exodus 20:2-23:19. These laws were what Israel agreed to obey when they made their first covenant (the “Old Covenant”) with God. It was a blood covenant, and half the blood was put on God’s altar and half was sprinkled on the people (Exod. 24:6-8).
“at the foot of the mountain.” The Hebrew text literally reads, “under” or “beneath” the mountain, but it means “at the foot of.” Saying “under the mountain” would be unclear in English. Here in Exodus 24:3-8, at the foot of Mount Sinai, the Israelites made a blood covenant with God that He would be their God and they would be His people and obey Him. But then just 40 days later Moses broke the tablets that had the Ten Commandments on them in the same place that Israel made the covenant only 40 days earlier (Exod. 32:9). The tablets with the Ten Commandments represented the covenant Israel made with God, and when Moses saw how openly and boldly Israel broke their newly-made covenant with God, he broke the tablets that represented the covenant.
“standing-stones.” It was a common practice to take a very large, long stone and set it upright as recognition of memorial of some event.
[For more on standing-stones, see commentary on Gen. 28:18.]