It is a good idea to begin a new paragraph here, as Revised Standard Version does. The word translated Only marks a transition, calling attention to what follows. It may be alternatively translated as “But this one thing,” “Just be sure that,” and so on.

Take heed and keep your soul diligently: literally “guard yourself and guard your soul earnestly.” Israel is addressed in the second person singular, but in many languages it will be more natural style to use the second person plural here (see verse 14). New Revised Standard Version provides a good model: “But take care and watch yourselves closely”; also New International Version “Be careful and…” or Contemporary English Version “be very careful.”

Lest you forget the things that your eyes have seen: lest you forget means “so that you will not forget” or “in order not to forget” (New Revised Standard Version). This clause should be bound closely with what comes before it: “… and make sure that you do not forget what you saw”—referring to God’s appearance to the people at Mount Sinai (Exodus 19–20). An alternative translation model for the first part of this verse is “So you must be careful not to forget the things you have seen God do.”

And lest they depart from your heart may also be expressed as “Keep on remembering them,” “Keep on reminding yourselves” (Contemporary English Version), or idiomatically as “Keep them always in your heart.”

All the days of your life may be rendered as “as long as you live” (Good News Translation). The whole sentence may be expressed as “For as long as you live, keep on reminding yourselves about them.”

Make them known: “teach them.” This theme recurs frequently in the book; see 6.7, 20; 11.19; 31.13; 32.46.

Your children and your children’s children: “your children and grandchildren.” No doubt great-grandchildren would be included also, should an Israelite live long enough to have them.

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 .