"For they kept your commandment": against idolatry. "and they observed Your covenant": of circumcision.
Variantly: "For they kept Your commandment": in Egypt. "and they observed Your covenant": in the desert, (the Levites, unlike the other tribes, having circumcised their sons in the desert).
(Devarim 33:10) "They shall teach Your judgments to Jacob": We are hereby taught that all rulings are by their mouths, as it is written (Ibid. 21:5) "And by their word shall every contention and every plague-spot be (ruled upon.)" "contention": These are contentions involving the (red) heifer, and sotah, and the heifer (of the broken neck). "plague-spot": those of men, houses, and garments.