What is the meaning of "Orach la'Tzadik Meisharim"?
Rashi: You, Hashem are a straight G-d; You straighten the Tzadik's path in order that he will receive his reward. The Tzadik is Yakov and his seed. The path is straight ways of conduct.
Radak: It means Orach Meisharim la'Tzadik. You will give to a Tzadik to prepare a straight path in front of him, and weigh his path.
Malbim: Before the Navi begins to complain about the serenity of the Resha'im, he explains that he does not ask like one who is unsure about Hashem's conduct. He knows that "Mishpetei Hashem Emes Tzadku Yachdav" (Tehilim 19:10). Rather, he comes to request that Hashem reveal the punishment of Resha'im. All prophets, when they spoke about this question, set this foundation 1 , that surely Hashem's ways are very good, just He should [reveal Resha'im's punishment] lest [seeing their serenity] cause people to veer from His ways. A Tzadik must go in straight ways, without doubting or complaining about the supreme conduct, just to believe that everything is straight.
Malbim: E.g. Asaf (Tehilim 73:1-2), Yirmeyah (12:1-2), Mal'achi (3:14-15).
Why does it say "Yashar Magal [Tefales]"?
Rashi: Hashem says, a Tzadik's steps will straighten his path. Pilus is an expression of contemplation - he sits and thinks in his heart which is better. So straighten his heart to lead him in the path of receiving reward for all that he conducted his deeds properly in front of You.
Radak: "Yashar" calls to Hashem. You are straight, and You straighten a Tzadik's steps. This is like Chazal said, if one comes to be Metamei himself, they open [opportunity] for him. If one comes to be Metaher himself, they help him (Shabbos 104a).
Malbim: When a Tzadik turns from the straight path, i.e. he investigates ways of [Hashem's] conduct and asks why Resha'im succeed, do not consider this a sin. You weigh with just scales and know his heart; he does not deny like one who departs from the path. Rather, he is like one who goes on a roundabout path in order to get to his destination. I am like one who believes and prays [that Resha'im not have serenity].
What is the meaning of "Tefales"?
Radak: This is straightness. The same applies to "Pales Magal Raglecha" (Mishlei 4:26). Therefore, the beam of a balance scale is called Peles, because it evens the weight. The same applies to "v'Shakal ba'Feles Harim" (below, 40:12).
