Albert Barnes' Notes on the Whole Bible - Psalms 89:2
For I have said - The Septuagint and the Latin Vulgate render this, “Thou hast said,” which is more in accordance with what the connection seems to demand; but the Hebrew will not admit of this construction. The true meaning seems to be, that the psalmist had said; that is, he had said in his mind; he had firmly believed; he had so received it as a truth that it might be spoken of as firmly settled, or as an indisputable reality. It was in his mind one of the things whose truthfulness did not... read more
The Pulpit Commentary - Psalms 89:2-4
The security of God's pledged word. "Thy seed will I establish forever." The keynote of the psalm is the "faithfulness" of God to his word. "God had entered into 'an everlasting covenant' with David, and had confirmed that covenant with an oath. In the most absolute and unconditional form, God had pledged himself to establish the kingdom of David and his seed forever, to beat down all their adversaries under their feet, and to maintain their throne as long as the sun and moon should... read more