Who are the "Rabim Omrim"?
Radak: They are the "Bnei Ish" (verse 3), "Rabim Omrim l'Nafshi" (3:3).
What is the question "Mi Yar'enu Tov"?
Rashi: Many Jews see Resha'im with wealth and serenity, and say 'who will see us in a good [state] - rich, and attaining desires like these people?'
Radak #1: Avshalom's rebellion should succeed; David will die, and Avshalom will reign over us. What is bad for me (David) is good for them, and they desire it!
Radak #2: This refers to "trust in Hashem" (verse 6). Many are concerned only for good in this world. I and those with me, our whole desire is the light of Your face in the world to come!
Malbim: During the famine, they said so - that rain will come, and Hashem will raise on them the light of His face and Hashgachah, which was lacking from them at the time of famine and Hester Panim.
What is the meaning of "Nesah Aleinu"?
Rashi #1: Raise for us the banner of the light of Your face. This is like "Harimu Nes", "Arim Nisi" (Yeshayah 62:10, 49:22).
Rashi #2: They are raised over us. This is like "Avnei Nezer Misnosesos" (Zecharyah 9:16).
Radak: Raise for us. The Sin is in place of Shin, and the Hei is in place of Aleph.
