Therefore - In view of all that Elihu had now said, be came to the conclusion that the views of Job were erroneous, and that he had no just cause of complaint. He had suffered no more than he had deserved; he might have obtained a release or mitigation if he had applied to God; and the government of God was just, and was every way worthy of confidence. The remarks of Job, therefore, complaining of the severity of his sufferings and of the government of God, were not based on knowledge, and had in fact no solid foundation.

Therefore doth Job open his mouth in vain - God will execute vengeance when it may best serve the ends of his justice, providence, and mercy. The delay of judgment is not proof that it shall not be executed; nor is the deferring of mercy any proof that God has forgotten to be gracious.
He multiplieth words without knowledge - However this may apply to Job, it most certainly applies very strongly and generally to the words, not only of Job's three friends, but to those also of Elihu himself. The contest is frequently a strife of words.

Therefore doth Job (h) open his mouth in vain; he multiplieth words without knowledge.
(h) For if he punished you as you deserved, you would not be able to open your mouth.

Therefore doth Job open his mouth in vain,.... In uttering such unbecoming expressions, observed, and refuted, in his loud complaints of God, and of his dealings with him, and in defence of himself;
he multiplieth words without knowledge; both against God and in answer to others; being in a great measure ignorant of the nature and number of his sins, and of his afflictions; and of the end of God in them, and of the right he had to lay them upon him; us well as of his duty patiently to bear them, and trust in God, and wait his own time for deliverance out of them; and or the truth of this he was afterwards convinced, and acknowledged it, Job 42:3.

Apodosis to Job 35:15.
in vain--rashly.