And you shall speak to him is not a command but an explanation of the plan to be followed. Thus you shall speak may be rendered as “You can speak” (Good News Translation) or “You are to speak” (New American Bible). And put the words in his mouth is literal, so “tell him what to say” (Good News Translation) or “tell him the words that he is to say” may express the meaning more clearly. And I will be with your mouth and with his mouth is also literal, using the same expression as in verse 12. What is meant is “I will help both of you to speak.”

And I will teach you what you shall do uses the second person plural in both cases. This can be made clear in English by saying “I will tell you both what to do” (Good News Translation). The word for teach, however, may suggest more than just telling them what to do; it implies some instruction in advance. In certain languages there will be constructions such as “I will inform you so that you will both know what to do.”

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 .