Seeing that is supplied by Revised Standard Version, which keeps verses 17-18 as one long question. It may be clearer to stop the question after “about to do?” and to begin verse 18 as a new sentence.

If a statement is used in the LORD’s reflection, it will be necessary to continue with a further statement: “Abraham shall become a great….”

Abraham shall become a great and mighty nation: this statement must often be adjusted to say, for example, “Abraham’s descendants will be…” or “Abraham will be the ancestor of….” Great and mighty nation: a form of this expression was used in relation to Abraham in 12.2. See also 17.20 regarding Ishmael. Mighty or “powerful” translates a word whose meaning is mighty in numbers, very large, and in such contexts as Exo 1.7, 20, “grew strong.” Speiser translates “populous.” However, Good News Translation, New English Bible, Revised English Bible, New Revised Standard Version have “mighty” or “powerful.”

And all the nations of the earth shall bless themselves by him: this statement is a parallel of 12.3, in which nations is replaced by “families,” and him by “you.” See discussion of 12.3.

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 .