Hast thou said - Thou hast said. In the original no question is asked. Nathan assumes, as far as words go, that the king has made this declaration. He wishes to draw forth a disclaimer.

And Nathan said, my lord, O king,.... He addresses him as with great veneration and respect due to his office, so as if he knew noticing of Bathsheba's application to him; and therefore begins and tells his story, as if the king had never heard anything relative to it:

hast thou said, Adonijah shall reign after me, and he shall sit upon my throne? surely it can never be, because of the notice which he himself had given him from the Lord, that one to be born should succeed him, plainly pointing to Solomon; and also because of the oath which he had sworn, to which Nathan was privy, that Solomon should reign after him; and yet if he had not given such orders, it was exceeding strange that Adonijah should presume to do what he had done.

And Nathan said, My lord, O king, hast thou said, Adonijah shall reign after me, and he shall sit upon my throne?

1:11-31 Observe Nathan's address to Bathsheba. Let me give thee counsel how to save thy own life, and the life of thy son. Such as this is the counsel Christ's ministers give us in his name, to give all diligence, not only that no man take our crown, Re 3:11, but that we save our lives, even the lives of our souls. David made a solemn declaration of his firm cleaving to his former resolution, that Solomon should be his successor. Even the recollection of the distresses from which the Lord redeemed him, increased his comfort, inspired his hopes, and animated him to his duty, under the decays of nature and the approach of death.