Wy does the Torah insert the (otherwise superfluous) wotd "va'Ani Hinei Lakachti es ha'Levi'im"?
Oznayim la'Torah (citing the Midrash Rabah): Ti teach us that Hashem's decisions to switch from the Bechorim to the Levi'im and to obligate the Bechorim to pay the Levi'im five Shekalim - for being rejected? - He carried out together wth His Beis-Din.
What does the Torah mean when it writes "Me'eis B'nei Yisrael"?
Rashi: It means that Yisrael will 'hire' the Levi'im 1 to serve in the Beis-Hamikdash instead of them.
What does the Torah mean when it writes "Tachas Kol B'chor ... "?
Rashi: It means that they merited this honor - that Hashem chose them - on account of the firstborn who were originally destined to serve, but who forfeited that right when they worshiped the Eigel, whereas they did not. 1
Seforno: It means that the Levi'im took over from the Bechoros by means of redemption (as the continuation of the Parshah will explain).
Oznayim la'Torah: And the Torah placed the fact before the reason - to teach us Emunah, that everything that Hashem does is based on sound reasoning and it is not necessary to query His motives.
What is "Vahayu Li ha'Levi'im coming to teach us?
Targum Onkelos and Targum Yonasan: It teaches us that the Levi'im should serve in the Beis-Hamikdash.
