“This is the line of Aaron and Moses at the time that God spoke to Moshe on Har Sinai / וְאֵלֶּה תּוֹלְדֹת אַהֲרֹן וּמֹשֶׁה בְּיוֹם דִּבֶּר הַשֵּׁם אֶת־מֹשֶׁה בְּהַר סִינָי / These were the names of Aaron’s sons Nadab the first born, Avihu, Elazar, Itamar / וְאֵלֶּה שְׁמוֹת בְּנֵי־אַהֲרֹן הַבְּכֹר נָדָב וַאֲבִיהוּא אֶלְעָזָר וְאִיתָמָר” (verses 1-2). Why does the introductory verse mention these are lines of Moshe and Aaron, but then go on and just mention the line of Aaron? Rashi explains that they were called the sons of Moshe because Moshe taught them Torah. This introduces a principle that anyone who teaches Torah is like they are a parent. The Gur Aryeh explains that the Kohanim are given this distinction and not all of Israel because Moshe taught the kohanim more. Another explanation is that later on the chapter will discuss the Leviim, and Moshe is representative of the non-kohen part of the tribe of Levi. Ibn Ezra focuses on the “on Har Sinai” part of the verse. The reason Moshe and Aaron are listed here is because the number of their offspring did not change from the time of the giving of the torah until now, in the second year. It was at that time that Hashem decided to give over the job of the priestly and administrative jobs from the first born to the tribe of Levi and the kohanim. Another possibility is that this verse is meant to connect to a verse back in Exodus. Back in Chapter 16 in the midst of the exodus story, there is a brief interlude in chapter 6 that discusses the lineage of the tribe of Levi. “These are the names of Levi’s sons by their lineage: Gershon, Kohath, and Merari; and the span of Levi’s life was 137 years. / וְאֵלֶּה שְׁמוֹת בְּנֵי־לֵוִי לְתֹלְדֹתָם גֵּרְשׁוֹן וּקְהָת וּמְרָרִי… וּשְׁנֵי חַיֵּי לֵוִי שֶׁבַע וּשְׁלֹשִׁים וּמְאַת שָׁנָה” (Exodus 6:16). What’s interesting there is that the lineage is not complete. It is simply a vehicle to get to Moshe and Aaron. “It is the same Aaron and Moses to whom God said, “Bring forth the Israelites from the land of Egypt, troop by troop” / הוּא אַהֲרֹן וּמֹשֶׁה אֲשֶׁר אָמַר הַשֵּׁם לָהֶם הוֹצִיאוּ אֶת־בְּנֵי־יִשְׂרָאֵל מֵאֶרֶץ מִצְרַיִם עַל־צִבְאֹתָם” (Exodus 6:16). Chapter 3 of Numbers then picks up from where that story left off. Again this another example of the transition between the exodus story and the upcoming entrance into the land of Canaan. The exodus story began with the lineage of Levi leading up to Moshe and Aaron, and now the entrance to the land continues that lineage link.