They did not know that Joseph understood them: that is, “The brothers did not know that Joseph understood what they were saying among themselves.” Some translations say “They did not know that Joseph understood their language” or “The brothers spoke in Hebrew and didn’t know that Joseph understood, because he spoke Egyptian and an interpreter….”

For there was an interpreter between them: this clause gives the reason why the brothers assumed that Joseph did not understand their language. Interpreter, which is used only here in the Old Testament, is based on a verb meaning “to be a spokesman.” Between them means that they were speaking to Joseph “through” or “by means of” an interpreter. In some languages interpreter is expressed as “word man,” “man of tongues,” or “he who turns their talk over.”

In translation it may be more natural to begin with the cause and say, for example, “Joseph was speaking to them through an interpreter, and so they did not think he understood what they had been saying.”

Quoted with permission from Reyburn, William D. and Fry, Euan McG. A Handbook on Genesis. (UBS Helps for Translators). New York: UBS, 1997. For this and other handbooks for translators see here .