Hard - The primary meaning of the original word is permanence (compare Deuteronomy 21:4; Micah 6:2). This may be applied as here to the hard dry rock, to running streams, or to stagnant pools. In either case, the idea is that of the barren dry soil, or the impassable marsh, in contrast with the fountain of life, carrying joy and refreshment with it.

Good understanding giveth favour,.... A good understanding in things natural and civil gives favour among men; and so a good understanding in divine and spiritual things gives a man favour among religious people, makes him taken notice of by them, and acceptable to them: and such an understanding no man has, unless it be given him; and such appear to have one that do the commandments of God, Psalm 111:10. The Israelites, for having and keeping the statutes of the Lord, were accounted by others a wise and an understanding people; and Christ, as man, when he increased in wisdom, grew in favour with God and men. It may be rendered, "good doctrine", as the Vulgate Latin version, or "right doctrine", as the Arabic version, "gives grace" (o); is the means of conveying grace into the hearts of men, and of increasing it. What if it should be rendered, "grace gives a good understanding" (p)? since it is certain, that an understanding to know God and Christ is a gift of grace, 1-John 5:20;
but the way of transgressors is hard; ungrateful and unpleasant to themselves and others; it is a rough and rugged way, in which they stumble and fall; and cannot walk with pleasure themselves, when their consciences are awakened, and they are loaded with guilt, and filled with terror; and must be very disagreeable to those who have seen the evil of them.
(o) "gratiam", Pagninus, Montanus. Vatablus. Mercerus. Drusius, Michaelis, Schultens. (p) "Successum bonum dat gratia", Junius & Tremellius.

The way of sinners is hard upon others, and hard to the sinner himself. The service of sin is slavery; the road to hell is strewed with the thorns and thistles that followed the curse.

Right perception and action secure good will, while evil ways are difficult as a stony road. The wicked left of God find punishment of sin in sinning.
hard--or, "harsh" (compare Hebrew: Deuteronomy 21:4; Jeremiah 5:15).

Four proverbs follow, whose connection appears to have been occasioned by the sound of their words (שׂכל כל, בדעת ברע, רשׁע רישׁ).
15 Fine prudence produceth favour;
But the way of the malicious is uncultivated.
Regarding שׂכל טוב (thus to be punctuated, without Makkeph with Munach, after Codd. and old editions), vid., p. 84; for the most part it corresponds with that which in a deep ethical sense we call fine culture. Regarding יתּן, vid., at Proverbs 10:10 : it is not used here, as there, impersonally, but has a personal subject: he brings forth, causes. Fine culture, which shows men how to take the right side and in all circumstances to strike the right key, exercises a kindly heart-winning influence, not merely, as would be expressed by ימצא חן, to the benefit of its possessor, but, as is expressed by יתּן חן, such as removes generally a partition wall and brings men closer to one another. The איתן [perennis], touching it both for the eye and the ear, forms the contrast to יתן חן. This word, an elative formation from יתן = Arab. wtn, denotes that which stretches itself far, and that with reference to time: that which remains the same during the course of time. "That which does not change in time, continuing the same, according to its nature, strong, firm, and thus איתן becomes the designation of the enduring and the solid, whose quality remains always the same." Thus Orelli, Die hebr. Synonyme der Zeit u. Ewigkeit, 1871. But that in the passage before us it denotes the way of the בגדים as "endlessly going forward," the explanation of Orelli, after Bttcher (Collectanea, p. 135), is withdrawn by the latter in the new Aehrenlese (where he reads ריב איתן, "constant strife"). And נחל איתן (Deuteronomy 21:4) does not mean "a brook, the existence of which is not dependent on the weather and the season of the year," at least not in accordance with the traditional meaning which is given Sota ix. 5 (cf. the Gemara), but a stony valley; for the Mishna says: איתן כמשׁמעו קשׁה, i.e., איתן is here, according to its verbal meaning, equivalent to קשׁה (hard). We are of the opinion that here, in the midst of the discussion of the law of the עגלה ערופה (the ritual for the atonement of a murder perpetrated by an unknown hand), the same meaning of the איתן is certified which is to be adopted in the passage before us. Maimuni
(Note: = R. Moses b. Maimum = Rambam, so called by the Jews from the initial letters of his name = Maimonides, d. 1204.)
(in Sota and Hilchoth Rozeach ix. 2) indeed, with the Mishna and Gemara, thinks the meaning of a "strong rushing wdy" to be compatible; but קשׁה is a word which more naturally denotes the property of the ground than of a river, and the description, Deuteronomy 21:4 : in a נחל איתן, in which there is no tillage and sowing, demands for נחל here the idea of the valley, and not primarily that of the valley-brook. According to this tradition, the Targum places a תּקּיפא in the Peshito translation of 15b, and the Venet. translates, after Kimchi, ὁδὸς δὲ ἀνταρτῶν (of ἀνταρτής from ἀνταίρειν) ἰσχυρά. The fundamental idea of remaining like itself, continuing, passes over into the idea of the firm, the hard, so that איתן is a word that interchanges with סלע, Numbers 24:21, and serves as a figurative designation of the rocky mountains, Jeremiah 49:19, and the rocky framework of the earth, Micah 6:2. Thus the meaning of hardness (πετρῶδες, Matthew 13:5) connects itself with the word, and at the same time, according to Deuteronomy 21:4, of the uncultivable and the uncultivated. The way of the בּגדים, the treacherous, i.e., the manner in which they transact with men, is stiff, as hard as stone, and repulsive; they follow selfish views, never placing themselves in sympathy with the condition of their neighbour; they are without the tenderness which is connected with fine culture; they remain destitute of feeling in things which, as we say, would soften a stone. It is unnecessary to give a catalogue of the different meanings of this איתן, such as vorago (Jerome), a standing bog (Umbreit), and ever trodden way (Bertheau), etc.; Schultens offers, as frequently, the relatively best: at via perfidorum pertinacissime tensum; but יתן does not mean to strain, but to extend. The lxx has between 15a and 15b the interpolation: τὸ δὲ γνῶναι νόμον διανοίας ἐστὶν ἀγαθῆς.

Giveth - Makes a man acceptable to God. Rough - Offensive and hateful to God and men, as rough ways are to a traveller.