Why does it say "Ki Hu Elokeinu"?
Radak: He judges our case against the nations.
Malbim: We understand Him because He is Elokeinu, with Hashgachah Pratis on us - not just because He created us. There is a connection between us - He is Elokeinu, and we are His flock.
Why are we called "Am Mar'iso Tzon Yado"?
Radak: He grazed us until here, and led us with the staff of His Emunah.
Malbim: He grazes us like one who tends his flock. He bestowed great benefits on us.
What does "ha'Yom" refer to?
Rashi: In this world [He will be Elokeinu] - if we will heed His voice.
Radak: Today He acquired us from Beis Avadim; we became "Tzon Yado", and He is Elokeinu.
Malbim: His connection with His [other] creations does not depend on their free choice. His connection with His nation depends on their choice. It is a new matter now - "Im b'Kolo Sishma'u."
Why does it say "Im b'Kolo Sishma'u"?
Radak: You will not be like your fathers, Dor ha'Midbar; right after He redeemed them, they tested Him in Refidim.