What are the connotations of the words "Tifk'du osam"?
Ramban: It means 'Remember and take note of them!' 1 In other words, Moshe was not just commanded to count Yisrael but to know their numbers by accepting their half-Shekalim as an atonement, so that they should not be subject to a plague that ensues when Yisrael are counted S'tam. 2
Targum Onkelos and Targum Yonasan: It means simply, 'You shall count them!'
Ramban: It is a Lashon of 'Hashgachah' (supervision), and is similar to the Pasuk in Bereishis 21:1 "va'Hashem Pakad es Sarah'.
Why does the Torah insert "Kol Yotzei Tzava" here?
Rashi and Ramban #1: To teach us that a man under the age of twenty cannot be conscripted. 1
Ramban: Perhaps this is because it is only when a ma turns twenty that he is strong enough to serve in the army, as the Mishnah states in Avos 'ben Esrim Lir'dof'.
Ramban: And the Torah adds "le'Tziv'osam, because each tribe comprised large hosts.
Why is Aharon mentioned here, but not in the census at the beginning of Ki Sissa