In Revised Standard Version verses 18-19 are a very long and complex sentence, which must be broken down into smaller units that will be easier to understand.

Beware lest there be among you: this is fairly unnatural English. Something like “Make sure that none of you” is better.

Man … woman … family … tribe: the various units of the nation. For tribes see 1.13.

Whose heart turns away … to go and serve the gods of those nations: this indicates a loss of loyalty and love, a betrayal or defection. See similar passages in 11.28; 13.6-7; 27.15. For gods see 4.28.

It is possible to state the first part of this verse positively as follows (Moses speaking):

• Make sure that every man, woman, family, and tribe here today remains faithful to Yahweh and never starts worshiping the gods of other nations.

The LORD our God: once more the first person plural; again the Septuagint has the second person plural.

Lest there be a root bearing poisonous and bitter fruit: this metaphor is fairly clear in meaning; Good News Translation has changed it to the simile “This would be like a root.” This refers to concealed betrayal, secret apostasy, whose results will become highly visible. Heb 12.15 makes reference to this passage. In some languages it will be helpful to begin the sentence with “If you do this you will be like….” Contemporary English Version translates “If even one of you worships idols, you will be like….”

Quoted with permission from Bratcher, Robert G. and Hatton, Howard A. A Handbook on Deuteronomy. (UBS Helps for Translators). New York: UBS, 2000. For this and other handbooks for translators see here .