For thou wilt prevent him. The change of the tense in the verbs does not break the connection of the discourse; and, therefore, I have, without hesitation, translated this sentence into the future tense, as we know that the changing of one tense into another is quite common in Hebrew. Those who limit this psalm to the last victory which David gained over foreign nations, and who suppose that the crown of which mention is here made was the crown of the king of the Ammonites, of which we have an account in sacred history, give, in my judgment, too low a view of what the Holy Spirit has here dictated concerning the perpetual prosperity of this kingdom. David, I have no doubt, comprehended his successors even to Christ, and intended to celebrate the continual course of the grace of God in maintaining his kingdom through successive ages. It was not of one man that it had been said, "I will be his father, and he shall be my son," (2 Samuel 7:14;) but this was a prophecy which ought to be extended from Solomon to Christ, as is fully established by the testimony of Isaiah, (Isaiah 9:6,) who informs us that it was fulfilled when the Son was given or manifested. When it is said, Thou wilt prevent him, the meaning is, that such will be the liberality and promptitude of God, in spontaneously bestowing blessings, that he will not only grant what is asked from him, but, anticipating the requests of the king, will load him with every kind of good things far beyond what he had ever expected. By blessings we are to understand abundance or plenteousness. Some translate the Hebrew word tvv, tob, goodness; [1] but with this I cannot agree. It is to be taken rather for the beneficence or the free gifts of God. Thus the meaning will be, The king shall want nothing which is requisite to make his life in every respect happy, since God of his own good pleasure will anticipate his wishes, and enrich him with an abundance of all good things. The Psalmist makes express mention of the crown, because it was the emblem and ensign of royalty; and he intimates by this that God would be the guardian of the king, whom he himself had created. But as the prophet testifies, that the royal diadem, after lying long dishonored in the dust, shall again be put upon the head of Christ, we come to the conclusion, that by this song the minds of the godly were elevated to the hope of the eternal kingdom, of which a shadow only, or an obscure image, was set forth in the person of the successors of David. The doctrine of the everlasting duration of the kingdom of Christ is, therefore, here established, seeing he was not placed upon the throne by the favor or suffrages of men, but by God, who, from heaven, set the royal crown upon his head with his own hand.

Footnotes

1 - Reading "blessings of goodness;" that is, the best or most excellent blessings.

For thou preventest him - Thou goest before him; thou dost anticipate him. See Psalm 17:13, margin. Our word "prevent" is now most commonly used in the sense of "hinder, stop, or intercept." This is not the original meaning of the English word; and the word is never used in this sense in the Bible. The English word, when our translation was made, meant to "go before," to "anticipate," and this is the uniform meaning of it in our English version, as it is the meaning of the original. See the notes at Job 3:12. Compare Psalm 59:10; Psalm 79:8; Psalm 88:13; Psalm 95:2; Psalm 119:147-148; Amos 9:10; see the notes at 1-Thessalonians 4:15. The meaning here is, that God had "anticipated" him, or his desires. He had gone before him. He had designed the blessing even before it was asked.
With the blessings of goodness - Blessings "indicating" goodness on his part; blessings adapted to promote the "good" or the welfare of him on whom they were bestowed. Perhaps the meaning here is, not only that they were "good," but they "seemed" to be good; they were not "blessings in disguise," or blessings as the result of previous calamity and trial, but blessings where there was no trial - no shadow - no appearance of disappointment.
Thou settest a crown of pure gold on his head - This does not refer to the time of his coronation, or the period when he was crowned a king, but it refers to the victory which he had achieved, and by which he had been made truly a king. He was crowned with triumph; he was shown to be a king; the victory was like making him a king, or setting a crown of pure gold upon his head. He was now a conqueror, and was indeed a king.

Thou preventest him - To prevent, from prcevenio, literally signifies to go before. Hence that prayer in the communion service of our public Liturgy, "Prevent us, O Lord, in all our doings, with thy most gracious favor!" That is, "Go before us in thy mercy, make our way plain, and enable us to perform what is right in thy sight!" And this sense of prevent is a literal version of the original word תקדמנו tekademennu. "For thou shalt go before him with the blessings of goodness."
Our ancestors used God before in this sense. So in Henry V.'s speech to the French herald previously to the battle of Agincourt: -
"Go therefore; tell thy master, here I am.
My ransom is this frail and worthless trunk;
My army, but a weak and sickly guard:
Yet, God before, tell him we will come on,
Though France himself, and such another neighbor,
Stand in our way."
A crown of pure gold - Probably alluding to the crown of the king of Rabbah, which, on the taking of the city, David took and put on his own head. See the history, 2-Samuel 12:26-30 (note).

For thou (b) preventest him with the blessings of goodness: thou settest a crown of pure gold on his head.
(b) You declared your liberal favour toward him before he prayed.

For thou preventest him with the blessings of goodness,.... Not temporal, but spiritual blessings, which spring from the grace and goodness of God, and consist of it; and relate to the spiritual and eternal welfare of those for whose sake he receives them, and who are blessed with them in him: his being "prevented" with them denotes the freeness of the donation of them; that before he could well ask for them, or before he had done requesting them, they were given him; and also the earliness of the gift of them, they were put into his hands before his incarnation, before he was manifest in the flesh, even from the foundation of the world, and before the world began, Ephesians 1:3, 2-Timothy 1:9, and likewise the order in which they were given; first to Christ, and then to his people in him, as the passages referred to show;
thou settest a crown of pure gold on his head; which is expressive of his victory over all enemies, sin, Satan, and the world, death and hell; and of his being possessed of his throne and kingdom; and has respect to his exaltation at the right hand of God, where he is crowned with glory and honour: and this crown being of "pure gold" denotes the purity, glory, solidity, and perpetuity of his kingdom; this is a crown, not which believers put upon him by believing in him, and ascribing the glory of their salvation to him, or what the church, called his mother, has crowned him with, Song 3:11, but which his father put upon him, who has set him King over his holy hill of Zion, Psalm 2:6; compare with this Revelation 14:14. The Septuagint and Vulgate Latin versions read "a crown of a precious stone"; and so Apollinarius; and seem to refer to the crown set on David's head, which had precious stones in it, 2-Samuel 12:30; Josephus (x) says it had a sardonyx. Fortunatus Scacchus (y) fancies the topaz is meant, and that the Hebrew text should be read "a crown of topaz"; mistaking the sense of the word "phaz", which never signifies a topaz, but the best gold, pure solid gold.
(x) Antiqu. l. 7. c. 7. s. 5. (y) Elaeochrism. Sacr. l. 3. c. 40. p. 1003.

preventest--literally, "to meet here in good sense," or "friendship" (Psalm 59:10; compare opposite, Psalm 17:13).
blessings of goodness--which confer happiness.
crown of pure gold--a figure for the highest royal prosperity.

(Hebrews.: 21:4-5) "Blessings of good" (Proverbs 24:25) are those which consist of good, i.e., true good fortune. The verb קדּם, because used of the favour which meets and presents one with some blessing, is construed with a double accusative, after the manner of verbs of putting on and bestowing (Ges. ֗139). Since Psalm 21:4 cannot be intended to refer to David's first coronation, but to the preservation and increase of the honour of his kingship, this particularisation of Psalm 21:4 sounds like a prediction of what is recorded in 2-Samuel 22:30 : after the conquest of the Ammonitish royal city Rabbah David set the Ammonitish crown (עטרת), which is renowned for the weight of its gold and its ornamentation with precious stones, upon his head. David was then advanced in years, and in consequence of heavy guilt, which, however, he had overcome by penitence and laying hold on the mercy of God, was come to the brink of the grave. He, worthy of death, still lived; and the victory over the Syro-Ammonitish power was a pledge to him of God's faithfulness in fulfilling his promises. It is contrary to the tenour of the words to say that Psalm 21:5 does not refer to length of life, but to hereditary succession to the throne. To wish any one that he may live לעולם, and especially a king, is a usual thing, 1-Kings 1:31, and frequently. The meaning is, may the life of the king be prolonged to an indefinitely distant day. What the people have desired elsewhere, they here acknowledge as bestowed upon the king.

Prevent - Crowning him with manifold blessings, both more and sooner than he expected. With - With excellent blessings.