What is the new song did Hashem put in his mouth?
Rashi: It is Shiras ha'Yam.
Radak #1: When He saved me, I composed a new song. He put it in my mouth, for He did for me Chesed for which I must thank Him.
Radak #2: He helped put a new song in my mouth, via Ru'ach ha'Kodesh. "Chadash" teaches that one is obligated to make a new song and praise for every miracle that Hashem does for him.
Malbim: Song to Hashem for a natural miracle is an old Shirah, for nature was already created. Miraculous salvation is a new act, and its Shirah is new. There are three gains from this. (a) Hashem's conduct of nature teaches that He created the world; this obligates honor. Seeing that He can change nature as He desires brings fear, due to reward and punishment. (b) Only Chachamim who investigate recognize Hashem from nature, but many see miracles, also with the eyes, not only via intellect. (c) Seeing miracles brings trust in Hashem; seeing His conduct of nature does not.
Why does it say "lEi'lokeinu" (plural)?
Radak: All who fear Hashem will praise and thank with him.
Malbim: "LEi'lokeinu" alludes to His Hashgachah Pratis over those who fear Him.
Will they fear and trust in Hashem only in the future?
Radak: Yes. Many who did not fear Him until now, when they see what He did with me, they will fear and trust in Him.
