«Do not add to his words»: This is part of the command about God’s words given to the Israelites in Deut 4.2 and 12.32. Revised English Bible expresses this as “Add nothing to his words”; Good News Translation expresses the sense in different words: “If you claim that he said something that he never said. . ..”

«Lest he rebuke you, and you be found a liar»: New Revised Standard Version uses more current English here with “or else he will rebuke you. . ..” These are the two consequences of disobeying the command in the first line. To «rebuke» is to speak to someone or be angry with them about their faults; see 3.12 where it is rendered “reprove.” It may also be expressed as “He will correct you” (Contemporary English Version). «And you be found a liar» is a rendering of the passive form of the verb “tell a lie” or “be a liar.” In English it may be expressed as “prove you a liar” (New International Version, Scott) or “show that you are a liar” (Good News Translation, Contemporary English Version). In translations in some other languages it is rendered “and everybody will know that you are a liar.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Proverbs. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .