For the Lord knoweth the way of the righteous - This is given as a reason why the wicked would not stand in the judgment with the righteous. The reason is, that the Lord, the great Judge, fully understands the character of those who are his friends, and can discriminate between them and all others, whatever pretences others may make to that character. Only those whom God approves, and loves, as his friends, will be able to stand in the day when the great decision shall be made. No one can impose on him by any mere pretensions to piety; no one can force his way to his favor, or to the rewards of the just, by power; no one can claim this in virtue of rank and station. No one can be admitted to the favor of God, and to the rewards of heaven, whose character is not such that it will bear the scrutiny of the Omniscient eve. Compare the notes at 2-Timothy 2:19. Man may be deceived in judging character, but God is not. When it is said that "the Lord knoweth the way of the righteous," the word "way" seems to be used to denote the whole of life - the manner of living (Notes, Psalm 1:1), and hence, the whole character. Perhaps there is included also the idea that the Lord knows the result of their manner of life - the issue to which it leads - and that, therefore, he can properly judge the righteous and assign them to that place in the future world, to wit, heaven, to which their actions tend.
But the way of the ungodly shall perish - The way or manner in which the ungodly live shall tend to ruin; their plans, and purposes, and hopes, shall come to nought. Their course, in fact, tends to destruction. None of their plans shall prosper in regard to religion: none of their hopes shall be fulfilled. In this, as in all other respects, they stand in strong contrast with the righteous, alike in this world and the world to come.

The Lord knoweth - יודע yodea approveth the way, "aloweth the way", Coverdale, of the righteous, צדיקים tsaddikim, from צדק tsadak, to give even weight; the men who give to all their due; opposed to רשעים reshaim, Psalm 1:1, they who withhold right from all; see above. Such holy men are under the continual eye of God's providence; he knows the way that they take; approves of their motives, purposes, and works, because they are all wrought through himself. He provides for them in all exigencies, and defends them both in body and soul.
The way of the ungodly shall perish - Their projects, designs and operations, shall perish; God's curse shall be on all that they have, do, and are. And in the day of judgment they shall be condemned to everlasting fire in the perdition of ungodly men. The wicked shall perish at the presence of the Lord. Reader take warning!

For the LORD (f) knoweth the way of the righteous: but the way of the ungodly shall perish.
(f) Approves and prospers, in the same way that to not know is to reprove and reject.

For the Lord knoweth the way of the righteous,.... The way in which he walks by faith, which is in Jesus Christ; the way in which he goes to the Father, and carries to him his sacrifices of prayer and praise, which meet with acceptance through him; the way in which he seeks for and expects justification, pardon, and salvation, namely, through the blood, righteousness, and sacrifice of Christ: and also it may denote his course, his walk and conversation; for the righteous man is a follower of God, he takes up the cross and follows after Christ: he walks not after the flesh, but after the Spirit, according to the rule of the word, and as becomes the Gospel of Christ: and this way of his in every sense the Lord "knows"; not merely as he is omniscient, for by his omniscience his eyes are upon the ways of all men; he knows the way of the wicked as well as the way of the righteous; but the sense is, that the Lord approves of and is well pleased with his way of faith and holiness; he knows this person, so as to love him and take delight and pleasure in him; his countenance beholds him with a smile; he is well pleased with him in Christ and for his sake, on whose account he has respect to him and to his offerings, to his service and duty, to his ways and works; and hence he is a blessed man, is in a happy situation, and all he does prospers, for he and his ways please the Lord: and hence also it is that neither he nor his way shall perish; the way he is in leads to everlasting life, and he being a follower of the Lord in a way pleasing to him, he shall never perish, but have eternal life;
but the way of the ungodly shall perish; for his way is a wicked way, the way of sinners, Psalm 1:1; it leads to destruction and death, and all that walk in it shall perish; for if is a way the Lord knows not, does not approve of, he abhors it; wherefore the man that continues in it will be unhappy, wretched, and miserable to all eternity. These last words therefore show the reason of the happiness of one sort of men, and the unhappiness of the other; and prove and confirm the same: the Lord knows, approves of, loves, and delights in the one; he does not approve of and delight in the other.

knoweth the way--attends to and provides for them (Psalm 101:6; Proverbs 12:10; Hosea 13:5).
way of the wicked--All their plans will end in disappointment and ruin (Psalm 37:13; Psalm 146:8; Proverbs 4:19).

Knoweth - He approves, loves, and delights in them, and therefore will recompence them. Perish - All their designs and courses shall come to nothing, and they shall perish with them.