I will bless her: this is introductory to the promise of giving Abraham a son through Sarah. The result of God’s blessing will be to give descendants to the barren Sarah. Bless means that God will cause good to come to Sarah, to make Sarah the recipient of his favorable action. Accordingly we may sometimes translate “I will be kind to Sarah,” “I will be good to Sarah,” or idiomatically “I will show my good heart to Sarah.” For further discussion see 1.22, 28.

And moreover I will give you a son by her: God promises that Sarah will bear Abraham’s son. This expression may also be rendered “I will even cause her to bear a son for you,” “I will even give you a son by means of her,” “She will have a son and he will be yours.”

I will bless her: this repetition introduces the more distant royal descendants that will follow. And she shall be a mother of nations: just as Abraham will become the father or ancestor of a multitude of nations (verse 4), so Sarah, his wife, will become a mother of nations. The translation of this expression should be parallel to that used of Abraham. See verse 4 for discussion. Some languages will prefer to use a term meaning “female ancestor” or one meaning “grandmother.”

Kings of peoples shall come from her: like the previous promise this one looks to the distant future. Sarah will become the ancestress of kings that rule over peoples or countries. See discussion in Gen 17.6. Kings of peoples may be rendered, for example, “rulers of people,” “chiefs who rule over their tribes.” Shall come from her is equivalent to “shall come forth from you” in Gen 17.6. The sense here is again “Among your descendants there will be kings who rule their people.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .