Then Moses said introduces the direct reply of Moses to the Pharaoh’s proposal in verse 28. Behold is omitted by many translations, but its influence may be expressed along with the words that follow, I am going out from you. The idea intended is “The moment I leave you” (New Jerusalem Bible). So New Revised Standard Version has “As soon as I leave you.” And I will pray to the LORD uses the same word as verse 28, so Moses agrees to pray to Yahweh.
It should be noted, however, that Moses does not add here the words “for you” as in verse 5. And the subject of the prayer will not be that the Pharaoh will have a change of heart. Rather Moses will ask Yahweh that the swarms of flies may depart. The word that is really the conjunction waw, so it is possible to translate as Durham does: “I am going straight out from your presence to pray to Yahweh, and the swarm of flying insects will leave.” This suggests that Moses has no doubt that the plague will be removed when he prays to Yahweh.
From Pharaoh seems a bit awkward, since Moses is speaking to the king. We should assume that this was a polite custom in the court of a king, to address the king in the form of the third person (as in 2 Sam 14.12). One may translate “from the king,” or simply change to the pronoun “you” and say “leave you” (Good News Translation). From his servants, and from his people may also be changed to second person singular, “your officials, and your people” (Good News Translation). This of course refers to the members of the kings’ court and to all the Egyptians.
Tomorrow refers to when “the flies will leave” (Good News Translation), not to when Moses will pray. Moses is so confident that Yahweh will hear his prayer that he even sets the time for the answer to take place. The word for tomorrow does not indicate any special time of day, so New Jerusalem Bible‘s “tomorrow morning” may be too specific.
Only is the same word used by the Pharaoh in verse 28. This introduces a condition and a warning that Moses now gives to the king. New American Bible uses “however,” and other translations simply start the sentence with “But.” Let not Pharaoh is still a part of Moses’ reply to the king. The meaning is “you must not.” Deal falsely again refers to the way the Pharaoh has previously changed his mind about letting the people go. Literally the Hebrew says “Only let not Pharaoh add to deceit.” This means “you must not deceive us again.” Contemporary English Version has a good model: “Only make sure that you’re telling the truth this time.” One may also express this as “… that you are not lying this time….”
By not letting the people go means to “prevent the people from going” (Good News Translation), or refusing to let them leave Egypt. To sacrifice to the LORD means “to offer sacrifices to the LORD” (New International Version), as explained in verses 26-28.
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 .