What God wants you to do is in Greek literally “the will,” a phrase current among the Jews when speaking of God’s will in an absolute sense.

You know may be understood in the sense of “you are acquainted with” or “you have learned,” emphasizing the results of having received the Law. In this way the first part of verse 18 becomes parallel to the second part.

To choose what is right (New English Bible “you know right from wrong”; An American Translation*, Jerusalem Bible “can tell what is right”) may have the force of proving what is best after one has put the various possibilities to the test (Revised Standard Version “approve that is excellent”; Moffatt “with a sense of what is vital in religion”; Phillips “truly to appreciate moral values”). It is important, however, in introducing this clause to avoid implying that the Jews habitually chose what is right. A more appropriate wording may be “you have learned from the law how to choose what is right” or “… how one ought to choose what is right,” suggesting that the Law forms the basis for teaching men how to choose the right, without necessarily implying that the Jews always do so.

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .