Why did he say "Hadricheni va'Amitecha"?
Radak: Guide me in the truth of Your existence. This is like Moshe asked "Har'eni Na Es Kevodecha" (Shemos 33:18). Regarding Derachecha (verse 4), it says Hodi'eni; it refers to Chachmah of nature. From it, man knows the Creator. It is like a ladder to knowing Chachmah of Elokus. From it, a person knows as much about the Creator as a body can know. A body cannot know that the truth of His existence like the Nefesh knows - "Ki Lo Yir'ani ha'Adam va'Chai" (Shemos 33:20). David asked Hashem to guide him in this, like one guides a toddler to walk little by little, until he goes by himself. Also Lamedeni is to accustom me, like Hadricheni. "K'Egel Lo Lumad", "Limud Midbar" (Yirmeyah 31:17, 2:24) are being accustomed.
Malbim: After asking to inform him conduct in Midos, he requested knowledge of truths hidden from human intellect; they are known only via Nevu'ah - "Sod Hashem li'Re'av" (verse 14). It has two parts. (a) Understanding matters of Elokus via investigation of intellect. (b) Knowing Torah laws and Mitzvos in deed. Hadricheni va'Amitecha applies to the latter, and Lamedeni applies to the former.
Why did he say "Atah Elokei Yish'i"?
Radak: You save me from afflictions of this world and give to me a heart to acquire the world to come, i.e. knowledge of the Emes.
Malbim: You save me in all needs of the body and Nefesh.
What is "Kol ha'Yom"?
Rashi: It is this world, which is day for the nations and night for Yisrael.
Radak: It is all days. The same applies to "Kol ha'Yom Chonen u'Malveh", "Kol ha'Yom Yevarechenhu" (37:26, 72:15).
What is the meaning of "Oscha Kivisi Kol ha'Yom"?
Radak: This is like "Shivisi Hashem l'Negdi Tamid" (16:8).
Malbim: I do not request You to guide and inform me for my benefit, e.g. honor - rather, my hope is to You. I request only means towards understanding You and serving You; this is the purpose of my hope. - "Elecha Hashem Nafshi Esa" (verse 1).
