1)
Why did Moshe not teach the princes together with the rest of the people?
1.
Razshi (in Matos, Bamidbar, 30b): He always taught them first in deference to their status.
2)
What is the Pasuk referring to? What did Moshe then command them?
1.
Rashi: Refer to 34:31:2:1.
2.
Ramban: All that Hashem spoke to him after the episode of the Golden Calf - from what was written on the second Luchos and all that was connected to the new covenant that Hashem was making with them. 1
3.
Seforno: It refers to before the episode of the Golden Calf - to the work concerning the construction of the Mishkan and those who served in it, and the Mitzvah of Machatzis ha'Shekel. 2