I have said. Some codices, with Septuagint and Vulgate, read "Thou hast said". Compare Psalms 89:19. The words of Ethan, reminding Jehovah of His covenant with David.
Mercy. Lovingkindness, or grace. Note "Mercy" (Psalms 89:2); "covenant" (Psalms 89:3); "seed" (Psalms 89:4); repeated below (verses: Psalms 89:19-32 and verses: Psalms 89:33-37).