tn The three first person verbs in v. 2a should be classified as cohortatives. The first two have pronominal suffixes, so the form itself does not indicate a cohortative. The third verb form is clearly cohortative.
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snI will bless you. The blessing of creation is now carried forward to the patriarch. In the garden God blessed Adam and Eve; in that blessing he gave them (1) a fruitful place, (2) endowed them with fertility to multiply, and (3) made them rulers over creation. That was all ruined at the fall. Now God begins to build his covenant people; in Gen 12–22 he promises to give Abram (1) a land flowing with milk and honey, (2) a great nation without number, and (3) kingship.
tnHeb “and be a blessing.” The verb form הְיֵה (héyeh) is the Qal imperative of the verb הָיָה (hayah). The vav (ו) with the imperative after the cohortatives indicates purpose or consequence. What does it mean for Abram to “be a blessing”? Will he be a channel or source of blessing for others, or a prime example of divine blessing? A similar statement occurs in Zech 8:13, where God assures his people, “You will be a blessing,” in contrast to the past when they “were a curse.” Certainly “curse” here does not refer to Israel being a source of a curse, but rather to the fact that they became a curse-word or byword among the nations, who regarded them as the epitome of an accursed people (see 2 Kgs 22:19; Jer 42:18; 44:8, 12, 22). Therefore the statement “be a blessing” seems to refer to Israel being transformed into a prime example of a blessed people, whose name will be used in blessing formulae, rather than in curses. If the statement “be a blessing” is understood in the same way in Gen 12:2, then it means that God would so bless Abram that other nations would hear of his fame and hold him up as a paradigm of divine blessing in their blessing formulae.