Melkor Manwe forgive

"Before the gates of Valmar Melkor abased himself at the feet of Manwë and sued for pardon, vowing that if he might be made only the least of the free people of Valinor he would aid the Valar in all their works, and most of all in the healing of the many hurts that he had done to the world. And Nienna aided his prayer; but Mandos was silent.
    Then Manwë granted him pardon; but the Valar would not yet suffer him to depart beyond their sight and vigilance, and he was constrained to dwell within the gates of Valmar. But fair-seeming were all the words and deeds of Melkor in that time, and both the Valar and the Eldar had profit from his aid and counsel, if they sought it; and therefore in a while he was given leave to go freely about the land, and it seemed to Manwë that the evil of Melkor was cured. For Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he; and he saw not to the depths of Melkor’s heart, and did not perceive that all love had departed from him for ever".

Silmarillion. CHAPTER 6: OF FËANOR AND THE UNCHAINING OF MELKOR