If integrated into the book of Daniel: 13.21.

If you refuse may be rendered “If you don’t” (Good News Translation), or even “If you don’t let us have our way [or, sleep with you].”

We will testify against you: The term testify implies making a statement under oath in some kind of judicial proceeding. Good News Translation separates the various elements in the word and translates “we will go to court and swear.” Good News Translation does not specifically express against you but that is clear from the context. Other possible approaches are “we will accuse you in court,” “we will publicly accuse you,” and “we will testify in court” (Contemporary English Version).

A young man was with you: This clause uses the same phrase “be with” as in verse 20. What they would accuse her of is having intercourse with a young man. Good News Translation does not express this accusation strongly enough by saying they would accuse her of being alone with a young man. That might be something improper enough to be shocking, but it would not call for criminal proceedings for adultery. Readers deserve to know that these judges are threatening Susanna with something very serious indeed—with death, in fact. See verse 41.

A possible way to restructure this verse is the following:

• If you don’t let us have sex with you [or, sleep with you], we will testify in court that we saw you send your servant girls away so that you could have sex with [or, make love with] a young man.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on The Shorter Books of the Deuterocanon. (UBS Helps for Translators). New York: UBS, 2006. For this and other handbooks for translators see here.