Knowest thou not this of old - That is, dost thou not know that this has always happened from the beginning of the world, or that this is the invariable course of events. His purpose is to show that it was the settled arrangement of Providence that the wicked would be overtaken with signal calamity. It was "so" settled that Job ought not to be surprised that it had occurred in "his" case. Zophar goes on to show that though a wicked man might rise high in honor, and obtain great wealth, yet that the fall would certainly come, and he would sink to a depth of degradation corresponding to the former prosperity.
Since man was placed upon earth - Since the creation; that is, it has always been so.

Knowest thou not this of old - This is a maxim as ancient as the world; it began with the first man: A wicked man shall triumph but a short time; God will destroy the proud doer.
Since man was placed upon earth - Literally, since Adam was placed on the earth; that is, since the fall, wickedness and hypocrisy have existed; but they have never triumphed long. Thou hast lately been expressing confidence in reference to a general judgment; but such is thy character, that thou hast little reason to anticipate with any joy the decisions of that day.

Knowest thou not this of old,.... Or "from eternity" (g), from the beginning of time, ever since the world was; as if he should say, if you are the knowing man you pretend to be, you must know this I am about to observe; and if you do not know it, you must be an ignorant man, since it is an ancient truth, confirmed by all experience from the creation; not that Job could know it so early, he was not the first man that was born, nor was he made before the hills, but was of yesterday, and comparatively knew nothing; but the sense is, that this about to be delivered was an old established maxim, of which there had been numerous instances,
since man, or "Adam",
was placed upon earth; referring to the putting of Adam in Eden to dress the garden, and keep it; and every man, ever since, is placed on earth by the ordination, and according to the will of God, where and for purposes he pleases: the instances Zophar might have in view are perhaps the expulsion of our first parents out of paradise, the vagabond state of Cain, the destruction of the old world by a flood, and of Sodom and Gomorrah by fire from heaven; which show that God, sooner or later, gives manifest tokens of his displeasure at sin and sinners, by his punishment of them for it. What he means is as follows.
(g) "ab aeterno", Junius & Tremellius, Drusius, Codurcus, Schmidt, Michaelis.

This - Which I am now about to say. Since - Since the world was made.