en_tn/gen/18/18.md

Should I hide from Abraham what I am about to do ... him?

God used this rhetorical question to say that he was going to talk to Abraham about something very important and that it was best for him to do so. Alternate translation: "I should not and will not hide from Abraham what I am about to do ... him." or "I should and will tell Abraham what I am about to do ... him" (See: rc://en/ta/man/translate/figs-rquestion)

Should I ... do, since ... him?

"I should not ... do. This is because ... him"

all the nations of the earth will be blessed in him

This can be stated in active form. Alternate translation: "I will bless all the nations of the earth through Abraham" (See: rc://en/ta/man/translate/figs-activepassive)

will be blessed in him

"will be blessed because of Abraham" or "will be blessed because I have blessed Abraham." For translating "in him" see how you translated "through you" in Genesis 12:3.