He said of course means “God answered.” But I will be with you is based on the verb “to be,” which is crucial to understanding verse 14. (See the comment there.) The tense is indicated only in context, so New English Bible‘s “I am with you” is possible. But this is certainly a promise to Moses as he undertakes his mission in Egypt. I will be with you may also be expressed as “I will be by your side,” meaning “I will go along with you and protect you.” (The “you” is singular.)
This shall be the sign for you is ambiguous. The word this may point back to the promise, “I will be with you,” or forward to the prediction, you shall serve God upon this mountain. Some scholars believe that this refers to the burning bush, in keeping with the normal understanding of sign as a supernatural phenomenon that confirms the truth of a divine revelation or promise. (But see the comment on “wonders” at 3.20.) Others suggest that part of the text is missing, possibly only a few words following the colon (), that described some supernatural sign.
Most translations try to preserve the ambiguity of the sign, or “proof,” but the use of the colon in Revised Standard Version and others definitely points to the prediction. New Jewish Publication Society’s Tanakh clearly points back to the promise by placing a period instead of a colon. Jerusalem Bible points to the missing words by using three ellipsis points. Good News Translation rearranges the clauses and changes this to “that,” thus allowing for either the promise or the prediction, or both, to be interpreted as the sign. This rearrangement will be a good model for translators. Other ways to express this shall be the sign are “that will be the proof,” “this [or, that] will show you that…,” or “this will be the evidence that I….”
That I have sent you (singular) is the fact to which the sign must point as proof of Moses’ divine commission. The use of the emphatic I may also be expressed as New English Bible and others have done: “This shall be the proof that it is I who have sent you.” Since the sign, or “proof,” is for Moses’ benefit, New Jewish Publication Society’s Tanakh has “this is the sign by which you shall know that I was the one who sent you,” or one may say “this will show for certain that I was the one who sent you.”
When you have brought forth the people out of Egypt is addressed to Moses in the singular. The people, of course, are the Israelites. Brought forth is the same word as in verses 10 and 11.
You shall serve God upon this mountain refers to Moses plus the Israelites, for the “you” is plural. (New English Bible has “you … all.”) To serve God means to “worship” God, but it implies some ritual act of worship. Since it is God who is speaking, Good News Translation more naturally has “worship me,” but there may be value in retaining God if “me” sounds too personal. Furthermore, this mountain was considered a holy place. It was Mount Sinai, the very place where Moses and YHWH were conversing.
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 .