# Now then, in the sight of all Israel ... all of you must
Here David is giving a command to all of the people of Israel. Alternate translation: "Now then, in the sight of all Israel, this assembly for Yahweh, and in the presence of our God, I command all of you to"
# in the sight of
"in the hearing of"
# keep and try to carry out
"carefully obey"
# your descendants after you forever
"your children and your descendants, who will take your place after you die"