1)
What is "Mishmeres Elokeihem u'Mishmeres ha'Taharah"?
1.
Malbim: It is like the Torah said - "v'Shamru Mishmartecha u'Mishmeres ha'Ohel."
2)
With the grammatical form of "ha'Taharah"?
1.
Ibn Ezra: It is like Chochmah, just the Nekudah under the Hei is switched from Shva to Kamatz, because Hei is a guttural letter.
3)
What was "k'Mitzvas David Shlomo Bno"?
1.
Malbim: The Torah did not fix which Leviyim will sing, and which will guard the gates. David and Shlomo fixed this. Therefore, Leviyim who serve in the Mikdash will have precedence over other Leviyim. Therefore, they appointed overseers to ensure that Ma'aser be brought to the Otzar, and given to Leviyim who stand in Beis Hashem.
