Did he see all Hashem's acts?
Rav Sadya Gaon: I contemplated them.
Rashi: This refers to matters given over to man.
Ibn Ezra: I saw the ends, and understood their source.
Ri Kara: I saw all the decrees that He makes in His world.
Rid: I saw that Hashem made the world in this way (refer to 8:17:2:4).
Metzudas David: I looked and contemplated.
What can man not find Tachas ha'Shamesh?
Rav Sadya Gaon: The secrets of things done under the sun.
Rashi: Man cannot understand Hashem's ways - the reward for everything done under Shamayim, for he sees Resha'im succeeding and Tzadikim descending.
Ibn Ezra, Ri Kara: One cannot understand this (Tzadik v'Ra Lo, Rasha v'Tov Lo 1 ). The Tzadik Iyov screamed about this.
Rid: He cannot find the end of what he engages in (Chachmah or wealth. Therefore, it is best that he eat and rejoice - 15.)
Metzudas David: The truth of the matters done in this world.
Ibn Ezra (14): All the verses from verse 14 until 9:13 discuss this.
What is "b'Shel"?
Ibn Ezra, Ri Kara, Metzudas David: "B'Shel" means due to, like "b'Sheli ha'Sa'ar ha'Gadol ha'Zeh" (Yonah 1:12).
Why does it say "Asher Ya'amol ha'Adam Levakesh v'Lo Yimtza"?
Rashi: I saw many toil to seek the end of Hashem's Midah, and they could not.
Rashbam, Ri Kara: Even if man will toil to know Hashem's Midos, he will not understand them.
Seforno: Sometimes he cannot attain an intellectual investigation, because it is too deep. Much time and diligent effort is needed; one who is distracted to attain excesses is unable to put such effort for Chachmah.
What is the meaning of "Gam Im Yomar ha'Chacham Lo Yuchal"?
Rashi: If you will say that a Chacham can know, this is wrong - even Moshe did not know - "Hodi'eini Na Es Derachecha 1 " (Shemos 33:13).
Seforno: Some matters are so difficult that even if a Chacham 2 will try to attain it via Drashos that he knows - if you heard old (Drashos), you will hear new (Brachos 40a) - he cannot, for 'one cannot tell to a man the power of Ma'ase Bereishis.'