I will be glad and rejoice in thy mercy - I will triumph and joy in thy mercy; that is, in the mercy which he had already experienced, and in that which he still hoped to enjoy. He had had abundant proofs of that mercy; he hoped for still further proofs of it; and he says that he would find his joy in that, and not in what idols could give.
For thou hast considered my trouble - In times past and now. He felt assured that his prayer would be regarded, and that God would relieve and deliver him.
Thou hast known my soul in adversities - In the troubles that have come upon me. That is, God had seen and known all the feelings of his heart in the time of adversity; his sorrow and anxiety; his hope and trust; his uncomplaining spirit; his feeling of entire dependence on God, and his belief that He would interpose to save him. God had not turned away from him, but had shown that he regarded with interest all his feelings, his desires, his hopes. It is much, in the time of trouble, to know that all our feelings are understood by God, that He sees all our sorrows, and that He will not be regardless of them. There are no states of mind more interesting than those which occur in adversities; there is no one who can fully understand the soul in adversities but God; there is no one but God who can entirely meet the needs of the soul in such seasons.