How often did the Kohanim need to ignite the fire on the Mizbe'ach?
Rashi (Yoma, 27b): Every morning, prior to the Tamid shel Shachar, 1 they arranged the Ma'arachah and ignited the fire.
In fulfillment of the Pasuk (7:5) "veha'Eish al ha'Mizbe'ach Tukad bo Lo Sichbeh" (Rashi, Ibid).
Seeing as fire descended from Heaven to burn the Korbanos, why did the Kohanim need to light a fire on the Mizbe'ach at all?
Rashi: Despite the fact that fire descended from Heaven to burn the Korbanos, 1 it is nevertheless a Mitzvah on the Kohanim to participate in the burning process.
See Sifsei Chachamim.
Why does the Torah say "[Bnei Aharon] ha'Kohen"? It should say "ha'Kohanim"!
Rashi: To teach us that Aharon may serve only when he wears his [eight] garments. The next verse says Bnei Aharon ha'Kohanim to teach that his sons may serve only if they are wearing their [four] Begadim. 1
Moshav Zekenim: Not everyone knows how to light the fire. The singular teaches that we need someone special, with this skill. (Refer to 1:7:3:1*.)
Rashi writes that that even though fire descends from Heaven, it is a Mitzvah for a Hedyot to bring fire. If so, Nadav and Avihu did properly! Why were they punished?