Why did he say ?ch Hu Tzuri"?
Radak: He alone is my Rock.
Malbim: Do not think that He will save me via natural means. Rather, He alone is my Rock and salvation, without a Shali'ach or via nature.
What is the meaning of "Lo Emot Rabah"?
Rashi: I will not have a major fall.
Rashi citing Shocher Tov: I will not fall to Rabah (i.e. Gehinom).
Radak: This is like "Ki Yipol Lo Yutal Ki Hashem Somech Yado" (37:24). The noun (Nefilah, a fall) is omitted, just like "u'Ma'achalo Beri'ah" (Chabakuk 1:16; its food is a fat Seh. Seh is omitted) and similar verses. Also, since it says Emot, one understands that it discusses a fall.
Malbim: "Misgabi Lo Emot" (my refuge that protects me from falling) is Rabah (great - it is Divine).
