Moses said to him introduces the reply of Moses to the king’s request. Aaron is not mentioned at all, and even Moses seems to ignore him by saying As soon as I have gone out of the city. The expression As soon as may be understood as “when” (New English Bible) or “as” (New Jewish Publication Society’s Tanakh), since it is simply a preposition attached to the Hebrew verb “I go out,” or “I exit.” New Jerusalem Bible‘s “The moment I leave” is suggested only by the context. The city was probably Rameses, mentioned in 1.11b (see the comment there on “city”) and again in 12.37. In contrast with the surrounding villages, the city and its temple area would have been protected by walls. Therefore out of the city would mean outside the walls and the protection of the city.

I will stretch out my hands to the LORD is an idiom meaning to pray, but it also describes the usual posture for prayer, with the palms of the hands open. Literally the Hebrew says “I will spread out [or, open] my [two] palms unto Yahweh.” Most translations retain the idiom, but it is good to add “in prayer,” as in Good News Translation “I will lift up my hands in prayer to the LORD.” The object of stretch out, however, is hands and not necessarily the arms. Durham translates “I will open my hands in prayer to Yahweh.” However, in a number of languages “hand” is included in the term for “arm” (by contrast see the comment at 4.6), so one may translate “I will stretch out [or, lift up] my arms and pray to Yahweh.”

The thunder will cease is literally “the sounds they will stop.” The plural form of the word “sound” is generally used for thunder in the collective sense. So one may express this as “the roaring sounds from the sky will stop.” And there will be no more hail is literally “and the hail will not be again.”

That you may know refers back to verse 14, along with several other verses that answer the king’s initial question in 5.2. (See the discussion there.) The singular you is used, meaning the king. That means “so that.” Moffatt has “to teach you that…,” and Translator’s Old Testament has “This will prove to you that….” The earth is the LORD’s is literally “to Yahweh is the earth.” This is the usual way to show possession, so “the earth belongs to the LORD” (Good News Translation). (See Psa 24.1.) Another possible model for the final part of this verse is “So that when the thunder stops and the hail stops falling, you [singular] will know that the earth belongs to Yahweh.”

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 .