The word For connects the warning in this verse with the demand in verse 3. In this context it means “or else” (King James Version). Good News Translation omits the word in order to place more emphasis on the if clause. If you refuse uses the participle form of the verb, which suggests the meaning of “If you keep on refusing.” Behold adds even more emphasis to both the demand and the warning, even though recent English translations tend to omit it as an archaic expression. (See the discussion at 1.9.)

Tomorrow is indefinite as far as the hour is concerned, in contrast to 9.18. I will bring is the causative participle, meaning literally “I am the one who will cause to enter.” The locusts were the migratory or desert type of winged, hopping in sects that swarm in the air. They were edible (Lev 11.22) but very destructive to plant life when they moved in large swarms. If locusts are unknown in a receptor culture, one may use a descriptive phrase for the first occurrence in Exodus—for example, “flying insects called ‘locusts,’ ”—and include an illustration of locusts for the readers. Into your country, of course, means Egypt.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .