With the pure - Those who are pure in their thoughts, their motives, their conduct.
Thou wilt show thyself pure - They will find that they have to deal with a God who is himself pure; who loves purity, and who will accompany it with appropriate rewards wherever it is found.
And with the froward - The word used here - עקשׁ ‛iqqêsh - means properly perverse; a man of a perverse and wicked mind. It is derived from a verb - עקשׁ ‛âqash - which means, to turn the wrong way, to wrest, to pervert. It would be applicable to a man who perverts or wrests the words of others from their true meaning; who prevaricates or is deceitful in his own conduct; who is not straightforward in his dealings; who takes advantage of circumstances to impose on others, and to promote his own ends; who is sour, harsh, crabbed, unaccommodating, unyielding, unkind. It is rendered perverse in Deuteronomy 32:5; Proverbs 8:8; Proverbs 19:1; Proverbs 28:6; froward here, and in 2-Samuel 22:27; Psalm 101:4; Proverbs 11:20; Proverbs 17:20; Proverbs 22:5; and crooked in Proverbs 2:15. The word does not occur elsewhere in the Old Testament.
Thou wilt show thyself froward - Margin, wrestle. In the corresponding place in 2-Samuel 22:27 it is rendered, "Thou wilt show thyself unsavory;" though the same word is used in the original. In the margin in that place, as here, the word is wrestle. The original word in each place - פתל pâthal - means to twist, to twine, to spin; and then, to be twisted; to be crooked, crafty, deceitful. In the form of the word which occurs here (Hithpael), it means, to show oneself crooked, crafty, perverse. (Gesenius, Lexicon). It cannot mean here that God would assume such a character, or that he would be crooked, crafty, perverse in his dealings with men, for no one can suppose that the psalmist meant to ascribe such a character to God; but the meaning plainly is, that God would deal with the man referred to according to his real character: instead of finding that God would deal with them as if they were pure, and righteous, and merciful, such men would find that he deals with them as they are - as perverse, crooked, wicked.

With the froward - עקש ikkesh, the perverse man; he that is crooked in his tempers and ways.
Thou wilt show thyself froward - תתפתל tithpattal, thou wilt set thyself to twist, twine, and wrestle. If he contend, thou wilt contend with him. Thou wilt follow him through all his windings; thou wilt trace him through all his crooked ways; untwist him in all his cunning wiles; and defeat all his schemes of stubbornness, fraud, overreaching, and deceit.
My old Psalter has, With the wiked thow sal be wike. Here the term wicked is taken in its true original sense, crooked, or perverse. With the wiked, the perverse, thou wilt show thyself wike, i.e., perverse; from to draw back, to slide. As he draws back from thee, thou wilt draw back from him. It may, as before intimated, come from to seek for enchantments; leaving God, and going to devils; to act like a witch: but here it must mean as above. The plain import is, "If thou perversely oppose thy Maker, he will oppose thee: no work or project shall prosper that is not begun in his name, and conducted in his fear."

With the pure thou wilt show thyself pure,.... None of Adam's posterity are pure by nature; they are all defiled with sin; and though some are pure in their own eyes, they are far from being cleansed from their filthiness; such only are pure who are sanctified by the Spirit of God, have clean hearts created in them, and whose hearts are purified by faith in the blood of Christ; who are justified by Christ's righteousness, and are washed from their sins in his blood; and who, in consequence of such grace, love, pureness of heart, speak a pure language, hold the mystery of faith in a pure conscience, and with a pure conversation, and live soberly, righteously, and godly: and whereas God is a pure and holy Being, his perfections, works, and word, are pure; he shows himself to be so to such persons, by providing for the honour of his purity and holiness in their redemption, sanctification, and salvation; by making all things to be pure to them; by granting them his presence, and blessing them with the vision of himself here and hereafter; see Matthew 5:8; this may likewise be understood of Christ, who, in his human nature, is pure from all sin, both original and actual: he indeed took upon him the sins of his people, and bore them, and made satisfaction for them, and brought in everlasting righteousness; which being done, God has showed himself pure to him, by justifying, acquitting, and discharging him from all such sins; by accepting his righteousness, and imputing it to those for whom he wrought it;
and with the froward thou wilt show thyself froward; or "thou wrestlest" (b), or wilt contend with them until they are destroyed: the same word is here used which Naphtali has his name from, Genesis 30:8. The froward are such as are of perverse dispositions, and of stubborn and obstinate tempers, and whose ways are crooked and distorted; and such were the people of the Jews in the times of Moses, and of Christ, Deuteronomy 32:5; and who seem here to be designed; even the Jews in Christ's time, who were just the reverse of the above characters, were cruel and unmerciful, faithless and hypocritical, filthy and pure, disbelieved the Messiah, rejected and crucified him, were contrary to God, and to all men; and therefore God walked contrary to them, as he threatened, Leviticus 26:27; the same as showing himself froward to them; for God is not froward and perverse in himself, nor in his ways, which are all equal, just, and pure: and though there is one and the same word used in our version, yet there are two different words in the Hebrew text; the same word that is used of the froward is not used of God; that which is used of God, as before observed, signifies wrestling, and designs God's contending with the people of the Jews, in a way of wrath and fury, which came upon them to the uttermost, and issued in their entire ruin as a people and nation; the words here had their fulfilment in the destruction of Jerusalem.
(b) "eluctaris", Junius & Tremellius; "colluctaris", Aben Ezra; "reluctaris", Gejerus; "certas", Schmidt.

froward--contrary to.

Pure - Free from the least mixture or appearance of unrighteousness, or unfaithfulness. Froward - Thou wilt cross him and walk contrary to him.