Why is "Choshech Amarav Yode'a Da'as"?
Rashi: One who knows Da'as, he does not speak many words.
R. Yonah: Da'as is what one knows, and it is stored in his hand. One who knows Da'as, and words of Chachmah are arranged on his lips, he speaks only what is proper in its time - "Davar b'Ito Mah Tov" (15:23). Also, he speaks only to an ear that hears, and in a place where they endear his words and drink them with thirst. He recognizes the attribute of Chachmah, the sweetness of its words, its value and desirability. Chazal said that silk is dear to those who wear silk and garment of Argamon [wool]. Therefore, they esteem and honor words of Chachmah. Do not cast pearls in front of pigs! Also, one who recognizes the attribute of Chachmah does not intend to glorify himself in people's eyes. The crown that Chachmah made for him suffices. Therefore, he speaks only where his words will yield fruits and Mitzvah.
Malbim: One who knows Da'as will not ask this (verse 26, i.e. why Tzadik v'Ra Lo). He knows that Hashem supervises, and that injustice does not come from Him, for He has the ultimate perfection. Surely His Mishpat is Emes! He does not investigate this question.
Why does it say "Yekar Ru'ach Ish Tevunah"?
Rashi: One who withholds words is a man of Tevunah.
R. Yonah: Tevunah refers to things that one must contemplate them with Binah. An Ish Tevunah is usually Yekar Ru'ach. He does not speak until he weighs on the scale of his Tevunah. He does not increase words, for he fears error. He answers questions only after deliberation and thought. He stood in the ways of investigation and knows the limit of man's understanding.
Malbim: An Ish Tevunah understands why Tzadik v'Ra Lo. It is to pay him for some sin that he did in this world, to benefit him in his end. He is Yekar Ru'ach - this question does not flare also in his inner Ru'ach.
